I too have found that the time frame is all over the map. Some places want an immediate response. Others are willing to wait. It's my view that most employers wait until they are well past the need before they actually hire someone. So, they need your acceptance right away because they need help right away. There's several things you can do with this, however:
- you can simply be honest and ask for more time.
- you use the tight time frame to pressure other potential offers.
- you accept but ask for a delayed start date