My band communicates pretty well in this area. We have when the solo should start in about every song. The length of the solo varies depending on what kind of mood I'm in. If I'm playing pretty sh**ty, then I'll signal the cutoff with a nod to the other guitarist. If I'm playing good, I feel like I oculd go on forever, and I probably would, but the other guitarist knows how long is long enough, and when he gives me a nod, I know I'm 8 bars from closing time.

If this doesn't work for you, then you could instead have your song set up like so (just an idea, not a rule)
Intro, Verse1, Chorus, Verse2, Chorus, [bridge (opt)], 16Bar Solo, Chorus, Outro.

You see, with this type of setup, you know that your solo starts after the bridge and ends 16 bars later. You should have no problem with the timing. It all come down to how much you've done it, and how well you know your band. Practice and you'll get better.