We put ours on when the beer gets down to a couple points above F.G. and its usualy active for about 2 days. It all depends on how many beers you have going at the same time. If all beers are about 2-3 days apart, you can probably get away with one as long as you have a shutoff valve on the blowoff tube so you can isolate the spund. Otherwise you'll have to replenish the head pressure with a C02 tank.
However, sometimes things don't go as planned and fermentations drag out so its not a bad idea to get a spare or 2.