This is a simple question with a complicated answer.
The Perhentian Islands on the northeast coast of Peninsular Malaysia may only be separated by a small body of water that is easily crossed by speedboat, but they're a world apart in vibe. For many, the question of how long to spend on the Perhentians is best answered with another question: how long have you got?
You can't talk about how long to spend on the Perhentians without talking about what the islands are like and, importantly, how they are different. You can snorkel from both. You can dive from both. You can hike and be chased into the ocean by monkeys on both (we've tested this personally on Kecil). At risk of wielding too broad a brush, Perhentian Kecil (the more northern, smaller island) is a party island, and Perhentian Besar, the southern, larger island, is more a sedate, family-orientated one.
People wanting to party
Now that we've got that out of the way, if you're rolling down the coast from Ko Pha Ngan, looking to continue the party and enjoy a vibrant singles' scene, then Perhentian Kecil is where you want to head, and, well, take as long as you need. The main party is on Long beach, with beach bars that routinely run into the early hours with plenty of booze on hand, and don't be surprised when you're offered illegal drugs. (Just in case you missed the memo, drugs like pot, cocaine and heroin are all illegal in Malaysia and if you are busted you've only got yourself to blame.) You could easily lose a week -- or a month -- here working up a massive hangover, sleeping till midday (if you can survive the generator electricity cut-off at 08:00), taking a swim, sleeping on the beach, and rinsing and repeating till the cows come home. But bear in mind you'll be paying 8+ ringgit for a 330ml can of beer (we'll not get into the price of drugs) which, at least at a backpacking level, works towards a rather expensive daily hangover.
If this is what you're after, bear in mind we were told by hoteliers that people, especially women travelling alone and having their drinks spiked at the Long Beach beach bars and then being attacked or raped, is an infrequent, though growing problem. We were told of a number of assaults this season and we were there only one month or so into season.
People wanting to dive
The Perhentians has a lot of dive sites of varying quality. The top three are Sugar Wreck (a large wreck), the Pinnacle (a, well, pinnacle) and T3 (a pile of boulders off the far side of Perhentian Besar). Keen divers will want to see all of them. Secondary sites include the Vietnamese Wreck and Shark Point among many others. If you just want to wing in, see the highlights and bail, two days would be ideal, but be warned, weekend diving around the Perhentians can get very, very busy. In April we spied at one stage 40 other divers in the water with us at the Pinnacle -- and that was midweek. Yes, it is a nice dive, no it wasn't so enjoyable sharing it with 40 others.
People wanting to snorkel
While the coral has been badly effected by El Nino, fresh water flush from the resorts and way too many boats, there are still plenty of fish, including lots of crowd pleasing favourites like bumphead parrotfish, Napolean wrasse, reef sharks and turtles. Some of the best off the beach snorkelling is at the far northern tip of Perhentian Kecil at D'Lagoon and then at their back beaches at Turtle Beach (watch out for the psycho monkey), Adam & Eve beach and the unnamed beach to the south of there. There is also good snorkelling in patches to the north of Coral Bay and to the south near Keranji beach. We've also heard good things about snorkelling to the west of Petani beach but haven't tried it in person. On Perhentian Besar, there is great snorkelling, with turtles at the aptly named Turtle Bay, sharks at Shark Point and some good coral off Bubbles Beach.
Snorkelling trips will generally take in some of the above, plus also visit spots like Rawa Island to the northwest of Perhentian Kecil. These trips come in half-day and full-day flavours and you could do a different one each day, giving you the best spread of snorkelling around the two islands.
Four to five days would be a comfortable stay with the kids on either island, though if you wanted to try both we'd suggest a longer stay. While both islands have calm and quiet beaches suitable for kids, Perhentian Besar is definitely the more family orientated of the two. PIR beach to the north, and Turtle Bay beyond it, are both outstanding excursions for families with kids old enough to snorkel. Also the jungle trail from PIR south to Teluk Dalam is doable with kids (not toddlers) and they'll have a good chance of seeing some wildlife - especially the giant monitor lizards in the lagoon just off Teluk Dalam. Another good spot with kids is Teluk KK, with a very calm and secluded setting. For older kids (teenagers), Perhentian Kecil may appeal as there are more activities there including banana boating and waterskiing. Both islands offer the above-mentioned snorkelling trips that would be great for kids. One more note: if you're after a family room in high season, book well in advance to be sure of being able to get one.
Other things to consider
There are no ATMs on the Perhentians. Bring enough cash with you. Some hotels and dive shops will take credit cards, but not all. If you run out of cash you can set aside half a day for the boat trip back to Kuala Besut then taxi to Jertih to get more dosh. If you're shuttling a lot between beaches by boat the costs add up, as can snorkel, mask and fin hire -- anywhere from 10 to 20 ringgit for a day of equipment hire can eat into a budget quickly. Some resorts and restaurants are dry (no alcohol) but beer and some wine can be easily purchased on both islands -- for a significant premium. Internet and WiFi availability is average, with restaurants and guesthouses often advertising it but in practice having sketchy quality at best.
The Travelfish newsletter is sent out every Monday and is jammed full of free advice for travel in Southeast Asia. You can see past issues here.