With the buff to gathering experience or something not long ago, you can probably smash through pretty fast. I know personally I jumped from 45 to 50 in just a few hours, when previously 5 level jumps took me much longer.
Quickest/easiest way, I think, would be hitting the leves with the evaluation bonuses, usually fairly easy to hit the +25% if you're geared well (plus the speed bonus if you clear under 5 minutes, also not hard).

Can't really speak much for 50+, but can't imagine it's too much different?