i change my strings when i see a lot of rust, which is quite long considering i always use Fast-Fret everyday, no matter how long i've been playing with it
Fast-fret works wonders. so far its the only string cleaner to prolong my string life for THIS long, but the only problem is that it leaves oil residue on the freboard. so when i change my strings i don't change them immediately, cause, well, i have to clean the board first