I usually respool about once a month. I never remove all the line, I usually pull off about 75-100 yards, tie onto my older line and fill from there. On my flipping outfit, I will pull much less off. You can get 2-3 refills on a spool of line.
For my braid, this will last me all season long. Since I use it for special applications (jigging in dirty water and froggin) I do not put as many miles on this stuff.