If I don't see/hear anything in 30 minutes I move a little bit. I usually am still hunting anyway, that's where I have the most luck. I go through the woods very slowly and as silently as possible, watching. You'll see them running around. Time of day is important. I don't typically hunt squirrel real early or real late like I would for deer. I see the most squirrel right now from about 9-11 and then 2-5 or so. Those seem to be the good times for the area I'm in.
-Jake