I haven't been able to locate any direct resource that addresses this problem, that, hopefully, does not occur too frequently. But, here are some concerns that I think would cause problems if you waited too long.
One of the biggest concerns is regarding a wedding that occurs close to twilight. This was a concern at my wedding and the mesader kiddushin (who, because he was a very close friend of the bride, came very early as he was part of the picture taking), informed us that the chuppah cannot occur during twilight, i.e. between sunset and nightfall. The reason is that there is doubt as to which calendar day that is. Furthermore, if the wedding was scheduled before twilight (in my case, it was), and you waited until after nightfall, it requires changing the date on the ketubah. As, in most cases, where the date is calligraphed on the ketubah, making this change is not that simple. So, in short, if the rabbi's lateness causes this problem, you can't wait past a certain point.
The next biggest problem that I see is chisaron mamon - monetary loss. The host is paying musicians and the caterer hourly and overtime usually has a minimum 1 hour charge. That can be very costly. The wedding expenses are high enough without adding unexpected hardship. Related to this, you may not even be able to go overtime if the caterer has booked another event after yours. Or, perhaps, the caterer has commitments from his staff to work only a certain number of hours. They've committed to that. The caterer and the staff, as well as the musicians, etc. have a right to go home to their families, or maybe they have other jobs after your event. So, you could be causing that staff chisaron mamon. In short, you need to ask the caterer what the situation is to judge how long you can wait.
Tircha d'tzibur - "burdening the crowd" could be another issue. Sometimes, many of your guests have other commitments. Some people have to attend other simchot on the same day as yours. Even if that's not the case, let's say you make everything outdoors in the summer when it's hot. (Yes, I think it's unpleasant when the host does that, but, it's their wedding, and I chose to come.) Guests are sitting in the heat, which they expect and can tolerate up to a point. If you keep them waiting around too long, you're unreasonably burdening them. In other situations, granted if things run late for them, they can leave early, but is that really what you would like esp. for close friends that help enhance the simcha? They want to be there as much as possible.
Along with these factors, you should consider the reason for the delay. These days, when you can reach the rabbi on the cell phone, you can at least get a better sense of how long the delay will be. (Was much harder pre-cell phone.) If the rabbi says, "I'm stuck before an accident scene with a train hitting a car, and it looks like it will take a few hours", well, then, at least, you know what to do!
The above are some, but not all factors that I can think of, offhand.