You generally have a couple of days to give them an answer.
If you have another potential job that you are close to perhaps getting an offer on (have already interviewed), you can call that 2nd job, tell them you have an offer from another company, and ask if they can expedite the process, because you do like them better. If they really want you, they will often try to do that. If you don't get a competing offer in that couple of days, you will then have to decide whether to go for the solid offer or turn it down in hopes of an offer at the 2nd company.
For a potential job that you haven't started the process on, or haven't yet had an interview for, you will have to decide: do you want the sure thing, or do you want to wait and try for the other job.
You can't make them wait a week or more for you to decide. They want someone who wants the job, and the longer you want to wait to decide, the less it appears you want the job. It is also unprofessional to take the job, and then continue looking for a job, being willing to jump from the first new job before or shortly after you start.