Too many variables to give a definitive answer. BigCatDaddy's right - when the flavor starts to fall off, when you have to crank up the power to get a good vape...
In addition to the type of topper, the juice and
vaping style affect service life.
I tend to clean my toppers every 2 to 4 fills, depending on size and flavor used. I'll generally replace at least the flavor wick - P&C yarn - at that time. Main wick if needed, or if I want to dry burn to clean the coil itself.