Getting so tired on this lol but hunting well for days and then find one and well one item and then its gone.. one item??
Why is it like this and is there just random how long it stays or what?

yeah it varies.. i also just had a one and done
but i got lucky and had one last at least 30 items yesterday

1-100 roll i belive

I think it's more than 100. Unless it was an event, but I had a 150+ once. Basically emptied my wellables in my vault with one well.

I just used 2 evil well maps and both poofed in 1

ye, roll allows over 100 uses. but in general most last under 10 uses, then the ones which do more often do quite a lot more

Random (RNG) roll of sustainability including Event modifiers.

If you just expect a Well to last from 1-3 items, then anything else is a reward.