49 yrs, 22 LFs
The biggest stones I got out was after my 13 th LF, and by the 22nd I'm still getting some stones, but mostly lots of chaff (that's suppose to mean one is getting to the last of it, I've heard)
Once you get to the chaff and then don't get any stones 3 LFs in a row, then it's OK to flush every 6 mos....