On equity options there is no average time to early assignment, it is dependent on lack of "time value" left in the option .
If your option bid price plus the stock price you wrote are lower even by a few cents than strike you are in the vulnerable zone.
On a few rare cases a stock I wrote options on, went into a free fall and I have been early assigned months before expiration to my advantage . This probably the short covering Robert explained so well above.
