It really depends on the culture of the company you are applying to...depending on your competitiveness as a candidate, they may give you all the time you need in making your decision...then again, if you wait too long, it could communicate indecisiveness or a lack of interest in the job.
Your best bet is during the interview process, ask the company how long you have to make a decision...most companies will recognize that you are probably interviewing for other jobs, and will give you a suitable window.
I went through the job interview process about 18 months ago...some companies required a nearly immediate response...others gave me a month to decide...sometimes you just have to take a gamble.