I believe there is a specific number for the maximum kw's that will go back in the battery with full regen but if say you are going down a very long hill and the battery starts to approach full, the regen will noticeably drop and you'll see less bubbles. This is most noticeable on our Prius(non PHEV) that has a relatively small battery. Even if the battery is fairly low at the top of a long hill, the battery will fill up and I'll have all but zero regen(engine breaking) by the bottom of the hill. As long as you have a larger EV battery and are not near full, I doubt you'd ever fill your battery up(and lose bubbles) by regen, even on a longer hill.
Someone else may know the exact max kw's of regen the Leaf is capable of, I don't.
2012 SL purchased used 2/'16
2013 S w/QC purchased new
Juicebox Premium 60a L1/L2 EVSE, Ebusbar 16a L1/L2 EVSE
'12 EVSEupgrade'd 20a L1/L2 EVSE, '13 EVSEupgrade'd adjustable 6-20a L2, 6-13a L1 EVSE
Zencar 13, 20, 30a L1/L2 portable EVSE
GE Durastation 30a