Feint lasts about 45 seconds, If it was 100% Chance to upgrade TH for 45 seconds, You could hit TH80 in that time frame with just a single THF <_>

I guess it could technically be "100%" with just feint on (I.E upgrades when Feint Lands)... But still, on a 3 minute timer, With multiple Thieves, You could drive TH up to insane amounts in a short period of time.