Yes, I believe you could use time as a guideline. You should notice that, with slow jelqing, you'll be doing about half or 2/3 of the repetitions you'ld be able to do if jelqing fast in the same amount of time (I hope this sentence made sense).
However, depending on the intensity of your jelqing motions (grip strength, erection level, etc.), you should look at how your penis reacts and look for signs that you should stop for that session. For instance, if I notice that, during the session, the dreaded red spots are starting to appear on my penis' skin, I stop and that's the end of my session for that day. It's one way my body has to tell me "enough already, you're pushing too hard".