There are great answers already here, but perhaps the most important consideration is making sure no one gets cut short when the trading is over.
Consider a 12-bar blues. It wouldn't really work for the soloists to trade off every 8 bars, because whoever goes second in soloing would only be halfway through his/her turn when the 12-bar form ended. (In particular, the first soloist would take bars 1-8, and the second soloist would take bars 9-12 followed by bars 1-4.) Spilling over like that can undermine the listener's sense of the form itself. Our ears generally expect to hear the soloist change at the very top/beginning of the form. If that doesn't happen, it feels uneasy, and we might get the feeling that we don't actually know where the top of the form is.
Jazz songs very frequently use 12-bar forms, 16-bar forms, or 32-bar forms. Letting the soloists trade off every 4 bars is great because all of those forms are evenly divisible by 4. Thus no soloist's turn will be interrupted by the end of the form. Trading 2 bars at a time is also common, and this frequently comes near the end of the trading, after having spent some time trading 4 bars. Trading 8 bars is also pretty common, although only if the entire song has an integer multiple of 8 bars.