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).

I concur with what Joe is saying in his answer above. Mashed potatoes are notoriously hard to hold for service, as they become gluey or pasty.

The methods Joe suggests for holding them are probably among the best possibilities. I will add these thoughts:

• Some people advocate putting a layer of melted butter or cream over the surface when using the oven holding method as outlined
• For moderate periods (say an hour or two), you can also use a slow cooker on low to hold, and serve them directly from the crock
• Mashed potatoes made from low-starch or waxy varieties (like the US Red Bliss type) tend to hold better than those made from high-starch (like US Russets) varieties--of course, there is an element of personal taste as well, as the variety of potato has a huge influence flavor and texture of the dish

While it isn't in your question, you may also wish to consider other dishes that hold much better or offer easier logistical challenges. A gratin type dish, such as potatoes anna, for example, has many virtues:

• many folks find it still delicious moderately warm or even room temperature
• it can also can be held for a moderate time in a warm oven
• more importantly, it can be prepared ahead, and then baked at the last minute without much other attention, and so might fit into your logistical plan more easily.
• its delicious, although certainly not the same as a good pile of mashpos, which I admit is my own personal favorite way to eat potatoes

...what happens when you boil them too long?

They take on water and turn to mush.

Not really what you want for great mashed potatoes.

I'm not sure what you mean by hold them but from reading the other answers it seems you want to delay making your mash. I let my boiled potatoes stand for some time, to get as much water out of them as possible before I make mine and by then they're usually cold anyway.

I use a potato ricer to make my mash potatoes and put them through the ricer whilst they're cold. I then microwave them back to hot, add butter only (no milk or liquid) and simply salt, freshly ground white pepper (not black) and a touch of freshly grated nutmeg.

I find this produces the smoothest, creamiest mashed potatoes you can achieve. You can also add some single cream, creme fraiche or soured cream for different flavours.

It all depends on how you like your mashed potatoes.

College students' open secret recipe for quick and lazy mashed potatoes casserole:

I microwave potatoes with 1 cm of water in a pyrex-ware for 6 minutes in a 1200w oven to get steamed potatoes. Poke holes or cut the potatoes in half to prevent splattering.

To get mashed potatoes, fill with water until half the depth of potatoes in the pyrex-ware and also 6 minutes. See, no worries about mushiness. Increase of decrease amount of water if you desire more/less fluidity of the mashed potatoes.

Making mashed potatoes this way also allows you to put in dill, cumin, whatever curry/spice mixture, cilantro, dried sliced shitake mushrooms and none of the water is wasted.

And then pour in a quarter bag of frozen green peas after mashing potatoes to cool the mashed potatoes down for immediate consumption. Could also throw in shredded smoked salmon or roasted chicken into the mix. Sriracha and Vietnamese fried onions-garlic notwithstanding.

Get a bun, the type they use as chili bowl, scrape the cavity to pour your mashed potato mixture in.

Yum, yum - better than ramen noodles.