Fastest reinstatment I've ever personally seen was about 8 months, although I have heard of some as short as 6 months.
The victim didn't miss any meetings right off the bat. Never made any waves. Always had her WT materials underlined in very visible highlighter ink. Turned in a "thank you" letter to the elders right off the bat letting them know she appreciated the spiritual discipline and was going to do her very best to restore her damaged relationship with Jehovah. Turned in a letter requesting reinstatment at 3 months. Was told to keep up the good work but continue to be patient. Turned in another letter around 6 months. Reinstated at around 8 months.
She seemed very sincere and cut to the heart.
I think an average time span if a person is sincerely trying to make a comeback, stays visible and maintains good communication with the elders and doesn't make ANY kind of excuses is probably 12-18 months.
As Farkel says: "It's all Bible Based" (tm).
om