According to
this page at the HP Lexicon, there are three or four classes a day. I guess four, or they wouldn't be able to have "double Potions with the Slytherins" in the afternoon.
Classes start at 9 am, there are two classes with short breaks in between. Then there is lunch and a break, and then classes resume at 1 pm. I suspect classes are a full hour long, or perhaps 50 minutes. So:
09:00 - 09:50 Class
10:00 - 10:50 Class
11:00 - 12:00 Lunch
12:00 - 13:00 Break
13:00 - 13:50 Class
14:00 - 14:50 Class
And then the school day is over. I could imagine it to go something like that; that's how I usually have it in my fics.