They whole point of WBTB is to get yourself to go to sleep right at the start of a REM cycle so that you are more likely to get lucid, either through a WILD or a more aware mind upon entering REM sleep. You only have so many REM cycles each night, so there's not an unlimited number of times you could do this, but if you want to do multiple WBTBs you'll have to figure out when your REM cycles are, which includes the WBTB. The reason people pick 4-5 hours into their sleep is because this is an easily defined time and the REM cycle is at least a moderate length at this time (15-20 minutes). I suggest, if you want to give it a go, figuring out your REM cycles on a typical night, using this chart as a guideline. Keep in mind that everyone is different do your times will be a little bit off, but that's a good starting point.

Also, don't do this if it means you lose a large amount of sleep. One of the keys to being successful at lucid dreaming is consistency. Consistent sleep schedule is a given, along with full sleep, so don't let this inhibit your sleep. Other than that, good luck and let us know how it goes!