When do mashed potatoes go bad?

2 days

1 week for me properly stored. After that ask yourself what cost more? The mashed potatoes or a doctor visit and days off work?

The usual safe time in a fridge for most foods is 3 days or so since a properly working fridge will keep foods outside the "Danger Zone" of temperatures for food (the Danger Zone is between 40 F or the temp of a fridge and 140 F or the temp of safely cooked fish and "Keep Warm" tables, etc).

Longer times in the Danger Zone than that and bacteria will proliferate "too much" for most bodies to handle easily--regular spoilage bacteria which could perhaps give a bit of GI distress, or not, but also pathogenic bacteria which would be much worse and doesn't necessarily cause the food to look or smell bad like spoilage bacteria.

(Some foods will last longer because they've been naturally preserved in one way or another--see those techniques here:

https://en.wikipedia.org/wiki/Food_preservation

...note that mashed potatoes won't generally be in treated in any of those ways.)

I doubt they taste fine.

Gross! They've for sure gone bad about a week ago!

They're probably okay. I would make them into pancakes w/ cheese & green onions, egg, flour and fry them...

I wouldn't eat it because bacteria has grown so much in it now. Once you cook any food and you want to save the food for later, first you'll have to cool the food down and put it in the fridge. You can keep any cooked food for up to 2 days in the fridge. The fridge is slowing down the bacteria growth, it would be worst if you left it out for days as it would reach the temperature danger zone and the bacteria would be already growing into the food.

Mmmmmmmmmmmmmmm... no.

I wouldn't eat them.

Three or four days in the fridge, alright, a week, maybe okay, but after that, rather not.

Okay, it's your risk, they may be good, but I wouldn't risk it. Make sure you have enough toilet paper supply and at hand when you need it.

Being mostly starch they don't really go bad that quickly.

That said, they kinda dry out if they're in the fridge for more than a day or two, so I'd eat them sooner than later.

Personally, if they look, smell, and taste ok, I would eat them.