doesn't recommend any interval, my Clymers manual suggests every year. Both are a little extreme.
I think a lot of it is by feel, but some things I noticed:
- front end was way spongy, especially on hard stops, and I could rock it at a standstill almost down to its stops
- bike became squirrely / wobbly on the highway
- bumps went from feeling firm to sharp and annoying
- front tire unevenly wearing/cupped
- the bike was no longer fun to ride
- mine became custom lowered - everytime I sat on it, the whole thing dropped.2 inches
Mine was an extreme example, as I'm pretty sure my springs were shot too. I plan on doing mine every other year with average riding.