you know, there's no formal "rule" for this. I think the best way is to use my rule of thumb:

The rule I use is "am I useful right now". If you can honestly answer "yes, I am useful to this torrent" then you should stay on it. If you're not, you might as well save the tracker some traffic.

It sounds like you're probably firewalled though, or something like that. If your connection acts like that, where you can't upload significantly (or at all, even), you are probably not useful to the torrent, and might as well drop off it.