I usually do my nails once a week (although I've been lazy lately!), so my top and base coat bottles tend to last me a long time. I usually never finish a full bottle though--I like to change it when my the formulas start to get very thick and take longer to dry. 

Not that often. My toes are always painted, but I only do my hands every other week? So maybe once a year or so.

It depends on the formula and how often I use them. I usually just use one bottle of topcoat (my favorite Seche Vite), and when I'm doing my fingers and toes weekly, it goes pretty fast. I don't usually make it through a whole bottle though--it gets too thick to use, or it gets too low for the brush to reach it. I would estimate every couple of months.