Rested Bonus Cap.

[Teleniel]

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.

[Flyojumper]

I know for a fact that it is more than a whole 3.3 levels. I created a lv1 at launch on a server I didn't play on and parked him in a rest zone. I came back to it around 3 weeks after and to test it I only killed monsters on the field, no quest, no mog hat, nothing but straight monster kills. He went from lv1.0 to 4.3 before using up his bar. I was expecting it to cap at 2 or maybe 3 levels, but it doesn't. It may just cap at 5 full levels or even higher.

For the record I also parked one of my char that was lv21 for roughly 3-4 weeks. When I started replaying with him, I went from lv21 to lv37 before I exhausted the bar. That is of course because quest exp, fates, etc don't use up nor benefit from the rested exp. But you can see how people can think the bar can go on for tens of levels. The only way to truly test it would be to use the rested exp bar solely on mob kills, like I did with my 1st test. It's not that hard really, just make a lv1 char on a server you don't play on, park him in rested area for a month (6 weeks+ to test better) then spend an hour or two leveling him up solely on mobs around the starting area. It takes planning but isn't really hard.

[Spanky]

Percentage based, cap is 100%.

So if you are empty 0/100,000xp on level, the bar will take up the entire thing. It's 100% of the bar...

Level 5 character will run through it in 2 levels of mob grinding. (6k xp)

Level 48 character will run through it in 2 levels of mob grinding (870k xp)

[Flyojumper]

Percentage based, cap is 100%.

So if you are empty 0/100,000xp on level, the bar will take up the entire thing. It's 100% of the bar...

Level 5 character will run through it in 2 levels of mob grinding. (6k xp)

Level 48 character will run through it in 2 levels of mob grinding (870k xp)

How do you explain that I went from 1.0 to 4.3 solely fighting monsters with no mog cap? I wasn't even chaining them that fast so I doubt any chain exp could have contributed to a full 1.3 levels.