I would say normally play it by 'ear'. That is when you get 'stale' in the full body workout, not progressing any further, then you are probably 'trained' enough to take on a split routine. Ironically enough, the beginning full body workout is normally a good way to get your body ready for consistent hard training. If you get to this point, congrats, as you have gone further in the 'journey' than many do.
-------------- Baseball is the only field of endeavor where a man can succeed three times out of ten and be considered a good performer