Not often enough 😂 I usually go by sound. When they start to sound too dull I change them

I usually run my finger under the string (between the fret board and string) and if a bunch of gray oxidized stuff comes off onto my finger tip I change it. There may be better ways to time it, but I notice they always sound brighter with this timing so I keep doing it.

Change?

I kept flatwounds on one of my guitars for about 12 years. Is this low time preference?