I re-spool when the level of the line begins to affect the reel's performance. I lose enough line to hang ups so that I have to re-spool often enough that the line stays in good shape. I use 10 pound test braided line (2 pound diameter) for general use.
In the reels that have been put away and not used I re-spool them when they are put back in service. The trout rigs are about due. They use 6 pound test Trilene.
Seventy years of chasing the elusive sunfish.