I'd prefer if Persist Spell just multiplied the duration by 10 (or a similar amount of increase) and was a +2 or +3 level adjustment. Anything on the round/minute/10 min/hour per level metric is simple enough, just bump it up to the next higher tier on that list. Anything lasting hour/level could be bumped up to 24 hour duration (I realize that makes it inferior to Extend by CL 13, but whatever). Anything lasting 1 day could be made day/level. Anything with a duration of X rounds or Concentration + X rounds could have the X multiplied by 10. Anything using 2 hours/level, day/level, concentration, or some other duration not mentioned above wouldn't benefit from the feat.
There, done.