My inner thigh muscle has gotten fairly weak, how long does it take typically to rehab it?

Depends. The answer to your question depends on the cause of the thigh weakness. If your weakness is secondary to a strain it could take up to six weeks or longer to rehab. If your thigh weakness is secondary to a nerve problem the weakness may not improve with exercise and could actually worsen. Best to see your physician to determine the cause of the weakness.
6-12 weeks. Any muscular atrophy or injury typically takes 6-12 weeks to rehab.