That depends on what you are going to do. If you are going to lift weights then do some stretches - each muscle should be stretched for about 30 seconds without bobbing or jerking the stretch. Then you can do reps with a light weight until the muscle is warm and loose. Personally within about 5 minutes I am warmed up, but for others it might be as much as 10 minutes.
I take about 5 minutes to warm up before running. I usually do some simple stretches to get the muscles loose. It is also important to do the same after your exercise routine, so you don't feel as sore later on. Believe me, it really helps!