I'm doing mine every 20,000 or so but it is a classic case of however often you feel necessary. More often is better but in my opinion as it is a secondary final filter then it shouldn't get too clogged too quick.
If you are using good fuel all the time then you may be OK, I have stretched it to 30k when doing the big lap as I couldn't get the bloody collar undone and didn't want to break it.
Originally Posted by Willstar
I big filled at every busy BP as I went around, no mystery soup added anywhere and I got away with it.