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.