I've been having RANDOM results - and it's motivating me to probably try building a coil soon...
I do NOT carefully track (I have several filled at any moment, and just swap em, fill em, etc) - but USUALLY I seem to go probably 3 or 4 fills, and then WASH. Don't dry burn, as I'm not really gunked up, just bottom of the barrell syndrom (some thick juice remnants and buildup of too many flavors). But I've had coils then last through several rinse-outs - meaning a couple weeks.
Then lately, I've had days when 2 or 3 went bad, that I THINK were on their first or second fill. So basically, inferior - duds. And they DIE - not fade - so I'm pretty sure it's an actual dead coil and not a cleaning issue.
As I said - I think I'm going to try to rebuild one soon!