You probably have a good feel for if your child NEEDS a limit. I have a DS9 and a DD5. My son started figuring out the computer about this age and was absolutely obsessed from day 1. We had to set time limits from almost the beginning because he would do it all day long. As a preschooler, I thought he should be doing a wide variety of things throughout the day. So even if he was doing "educational" stuff, I limited it.
My DD5 though is really just moderately interested in the computer and has never needed a limit to this point. I let her come and go from the computer. She has always gets off on her own in a reasonable amount of time.
