This completely depends on you and the program. Some people recover more quickly than others and can go with shorter breaks. Your rest time is tied to the intensity of the exercise. If you're just cranking a few quick sets of situps, maybe you need 30 seconds in between. If you're maxing or doing very high rep sets, you can wait anywhere from 2-5 minutes or more.
I believe 3-5 minutes minimum is the time it takes for the nervous system to recover after a very heavy set.
To give you something that actually works... just pick a number and see how you feel and adjust accordingly. Start with 60 seconds. Shorten if you feel ready, take a little longer if you aren't. Try not to go much over 90 seconds unless you're maxing or doing long sets/supersets.