A typical wedding reception is 5 hours long. One hour will go to the cocktail hour. That is when the bride and groom are taking pictures with their bridal party. The remaining four hours will be the grand entrance, the important dances, dinner and the open dancing.
A properly set up dinner will run from 45 minutes to an 1 hour 15 minute event. During that time the bride and groom will normally go table to table to thank the guests individually and be able to chit chat with their guests. Usually at this time, the photographer will be taking photos of the guests and the bride and groom at the tables. I have seen the bride and groom eat at the center table and the guests were going up to them and taking photos. Most of the bride and grooms opt to go to the tables. What usually happens is the bride and groom do not get a chance to eat their whole meal since they are visiting all the tables. Please make sure you get to eat since there is plenty of dancing and wedding to enjoy.
Keep in mind that if the wedding has a buffet it will take a little longer since folks are serving themselves and being excused table by table than if the waiters are distributing the food. Have the dj or the wedding planner excuse the tables if not people will help themselves thus slowing the process.