This topic is widely debated on many detailing forums - it will depend on the wax that you are using partly, but I would personally not go for anything more than three layers in succession (applied 24hrs apart). You can, with most waxes, put on as many layers as you like but as you say there comes a point where you are simply wasting your time in doing so... what I would personally do is when you come to doing a new layer, wax just one panel and compare with an unwaxed panel to see if you can see a difference - if so, do the whole car and if not, stop.