that totally depends on what you will be doing. we did our first dance. had a sit down dinner. after dh and i ate we walked around to every table to talk to guests. (6 tables) then we cut the cake and played the shoe game while the kitchen was cutting the cake to serve to the guests. we ate the cake and then had a time to thank our parents and honor the couple that was present that had been married the longest. then we left.
we didnt have any dancing beyond our 1st dance because we knew it would have offended some guests. so our wedding reception wasnt as long as most peoples.
if you are gonna do a lot of dancing, then take that into consideration. also, if you are having a buffet of any kind (dinner, dessert, candy..) then add additional time for eveyone to go through.