started cooking for the homeless How long would it take to boil 10kg of potatoes to make mash potatoes

If you haven't already, search starting with "cooking for a crowd" on the web, or look at professional cookbooks for volume cooking.

It depends on a lot of things, most obviously the size of the potatoes and the size of the pot, but also the type and condition of the potatoes, altitude etc. You could start checking at 20 min, but it could take twice that.