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.