Being on the other end of waiting for a reply, I give people 10 business days to return a signed offer letter (from the date of receipt) - and that is stated on the offer letter itself.
However, before sending that letter, I do give a verbal or email offer first. I prefer to hear back from someone within a week to that verbal/email offer.
As SM applicants, we want the best job financially, emotionally and artistically fullfilling-ly that we can get. So we delay responding, and sometimes the employer is fine with that. However, I've been in the situation twice where I offered a job and the person I offered it to put off the response for a week, then another week, then another - then it gets to the point where it is difficult for the employer (me) to find someone of equal or greater quality to fill the position because it's too close to the start date.
I'm presently in a situation where we offered a job to someone, they said they would respond the next day - then they said they would respond by that Friday. When we heard nothing on Friday, we contacted them the following Monday and they asked again for the following Friday as a new deadline. The job starts in 2 months. I said I was opening up the position, and they could still tell me Friday, but I had to start looking. At this point, I am already 3 months behind on the search.
1 week is my new limit, as an employer. As a job hunter, I should probably take my own preference into consideration and make a personal rule to respond in a week or less.