How many days can i safely use flonex to reduce post nasal drip? can i use it for several weeks or is there rebound effect?

As long as you wish: There are little side effects associated with Flonase which is intended to be used on a daily basis despite some concern over cataracts. Unlike Afrin, (oxymetazoline) it has no rebound effect but one would expect the allergic symptoms to return within days after cessation. Be sure to follow direction otherwise you may end up with a nosebleed.( which may happen for a few even if you follow direction)