Assuming we're talking about NLHE cash games (limit is very different), this depends on at least:
1. The number of outs (15 outs on the flop are actually favorites over one pair hands)
2. How aggressive you are (can you get the better hand to fold if you don't catch your draw?) - if you're passive and just call, the better hand will not fold, so you have to catch to win the pot
3. Your implied odds
(if you hit your draw, will villain put more money in after you hit? Is your draw obvious, like a flush draw, or is it hidden, like some straight draws?)
I bet draws when I think the combination of the above is in my favor, especially if I raised preflop. I call draws if I think Villain is passive and bets small - but not always, sometimes I raise in these situations if I think I have fold equity (i.e., if I think Villain might fold) - and I think I shouldn't call as much as I do heads up (multiway pots are different - there's a fourth "it depends" I didn't think of until now).
Oops, here's number 5 - your position. Don't draw out of position unless you're the aggressor.