the +50% rested XP bonus is based of %level... so level 49>50 goes a lot further than level 1>2 (take this in to consideration when "using" your rested XP.
Rested XP bonus applies to white-XP, that is it won't give you a bonus to leve, fate or dungeon reward XP, or bonus XP for crafting a new recepie / gathering a new item. It will also not give you a bonus toward Chocobo-XP.
It will give you a bonus to crafting actions (finishing a synth), gathering actions (harvesting an item), or basic enemy XP.

All that being said it is very hard to determine exactly how much rested XP you can have. It does take a while to accure, and often you are earning XP that does not gain the benefit making it hard to measure. I would say it can be at least 1 full level, probably isn't more than 3 full levels (although maybe up to 5?) but it doesn't apply to *most* of the big XP you earn, and is not a multiplicative bonus.