How long does one can of spray paint typically last?

Depends on how many coats you need to do.

1 can of silver spray paint with 1 layer on a pendleton bottle covered 15 bottles. I may have gone a little heavy with it, but no drips.

1 can of white spray paint, 2-3 layers on a sky vodka bottle, covered 10 bottles.

I'd say buy 3-4 cans for 30 bottles.