Hi Sue
Time required will depend very much on what you want to find out. If you are not a teacher in the education sector you wish to observe, you should probably allow longer. As to number of sessions, it depends what you are looking at - a particular topic area, subject, lesson, series of lessons - or whether your focus is more on the discourse space of the classroom (i.e. behaviours, relations, rather than content). Two good books (quite short) to read are:
Using observations in Small-Scale Research: A Beginner's Guide by Mary Simpson and Jennifer Tuson.
Participant Observation by James P Spradley.