Different types of trains take different times due to the different speeds they travel, but I'd figure on about 6 hours from Ayutthaya to Phitsanulok by train, assuming no major delays. The diesel rail cars are a bit faster most of the time, but cost more too. Trains leave the original station on time, but are almost always later and later as they work their way down the line. You can see the schedule for trains from Ayutthaya to P-lok at this link: http://www.railway.co.th/English/Time_HTML.asp
The bus from P-lok to Sukhothai is about an hour. Hope that helps. Cheers.