(I'm sure that different communities vary wildly, so I'll answer for the one I'm most familiar with, the Chabad community. Even in narrowing it to this, there will still be quite a bit of variation depending on locale, time of year, etc. If it's just the rabbi that's Chabad, and not the family, these rules will likely not apply.)
TL;DR
A Chabad wedding can last between 3 and 6 hours, and occasionally more. You are not required to stay all the way.
There are four main parts to the wedding. Kabbalas Panim, Chupa, dinner, and dancing. Some people stay for all four, others, for just the second or third, and still others come for just the fourth. From start to finish, a wedding can run from 3 to 6 hours. Generally, family and close friends stick it all the way out, while others (especially those with children) often leave earlier. There are exceptions to both, and you're unlikely to offend anyone either way.
Kabbalas Panim is the reception. Usually held in two separate rooms, one each for men and women. Light refreshments are often served, the groom will repeat a maamar and the tenaim are read.
Chupa! is the actual wedding ceremony. Very solemn until breaking the glass, then very happy. Usually takes place outside, and lasts anywhere from 20 to 60 minutes, depending on a number of factors (location, singing, weather, etc.) You won't get a chance to speak to the groom or bride, so if you want to wish them "Mazel Tov!" you'll have to stick around for the next part.
Dinner! !רצה לבנינו, על כן התחתנו, בוא נאכל
While the bride and groom head to the yichud room to break their fast, you'll head to the dining hall. May be just inside the building, down the block, or across town. Usually one of the first two. Dinner feels like it must be between 45 and 75 minutes (I've never checked); the groom and bride will come out near the beginning. This may be your chance to wish them "Mazel Tov!" Often, the groom will go around the men's side with a bottle, offering people a L'chaim.
Dancing! The part we've all been waiting for. As the band kicks up, everyone will head to the middle of the room for some serious hora. Ok, fine, it's not the hora. This part can get a little crazy, and by now you've probably lost your chance to speak to either of the couple, unless you're willing to stick it out till benching at the end. Dancing can run anywhere from 2 hours to 4+ (people will drop out of the circle throughout to rest and refresh). Since Chabad weddings usually start mid to late afternoon, the dancing often ends between 11 and 1 o'clock. There is no obligation to stick it out, and often by the end of it, many of the people remaining are relatives and close friends. Often there are bochurim, and others, still around, waiting to bentch and say sheva brachos at the end.
Note: This is my experience, garnered from the weddings to which I've been. Feel free to correct anything I've gotten wrong or missed, or to broaden it to include your experience (for example, I have little knowledge of the kabbolas panim and what's going on on the women's side). If you feel this is similar enough to other customs and want include those, go ahead; just make sure to edit in which groups it applies to.
