2-3 days, Bob?
I've kept partially used onions in the fridge on a plate with plastic wrap over them for more than a week and they were fine. Tasted good, not slimy, nothing growing on them (that you could see
).... I always buy big onions and it seems I always have one in the fridge for a while. I don't throw it out until it starts to dry up and the layers separate. Not that it's a good thing, but I thought they lasted longer than a couple days.....
I keep my onions on the fridge in the plastic bag, but the bag is opened and loose so the air can get at them. I wish I could get a hanging basket, but my kitchen is layed out so goofy I have no idea where I'd hang it. The only cool, dark place in my house is the basement