A thigh strain or quadriceps strain is a tear in one of the quadriceps muscles at the front of the thigh. It can range from a mild discomfort to a full blown tear of most of the muscle resulting in severe pain and inability to walk.
The breakage of the stitches starts within 2 weeks of the stitching but if you want the complete stitches to disappear then the process of hydrolysis or the enzymes in our body do that in a period of 60 days and therefore it approximately takes up to 2 months for the stitches to get dissolved.
Normally it takes anywhere from 2-6 weeks, depending on the sutures that were used. For dissolve-able stitches it take around 2-3 for it to be fully closed. Your best bet is to ask your doctor because size of the scar could prolong recovery.