Thresh's Flay (E) has a usefull passive:
Thresh's basic attacks deal bonus magic damage on each hit. This value is equal to the total number of Souls collected, plus a percentage of his attack damage based on the amount of time since his last attack.
When does the charging start?
How long does it take the passive to be fully charged?
Does the charging scale with the attackspeed? (more AS = less chargetime?)