Need Converter freshened

Seems like a good deal considering the work involved. Is there a rule of thumb for how often it needs to be freshened?
Definitely not. As a procrastinator I would wait too long and have it cost me a round. There are signs to look for in your racing performance such as inconsistent launch performance, rpm higher at finish line with same e.t's or gradually slowing e.t's with no apparent changes in motor performance. I'm sure there are others. Power level and frequency at the track as well.