Can you boil the potatoes for mashed potatoes too long?
Yes, you can.

The problem is that the potatoes start absorbing water, and you end up with a really runny mash that's difficult to flavor (as most people flavor their mash by the addition of flavorful liquids).

update : There's a few things that you can do to hold them.  The first is to drain the water, and leave the potato bits (unmashed) in the pot.  If you leave them uncovered, they'll cool off, but it'll also let moisture evaporate, which can help to fix some of the problems I mentioned earlier.

You don't want to let them cool off too far, or they'll not mash up smoothly; if you're going to need to hold them for more than 5-10 minutes, put a lid on them.  Once it comes time to mash, refresh them with whatever liquid you're going to add to the mash, warmed.

If you're going to need to hold them a really long time, you have two options ... one is to mash them but intentionally reduce the liquid, and then refresh them with more hot liquid a few minutes before serving.  The other is to transfer them to a baking dish after mashing, and put it in a low oven.  (if you rough up the surface, and put it under the broiler right before serving, you can get a crust on it for a bit of variety).