Many studies have been done trying to answer these questions. Can an exercise program help athletes avoid surgery? How long does it take? What kind of exercise program is needed? Is an exercise program even needed? Perhaps it's possible to return to full strength given enough time.
A recent study from a sports lab in Greece compared men with an ACL injury to a group of healthy but very active men. The ACL group did not follow any kind of specific or structured exercise or rehab program. They managed their injury with the old standby approach: Rest, Ice, Compression, and Elevation (R.I.C.E.).
Muscle testing before and after showed that their strength slowly came back but not to the preinjury level. The hamstrings recovered more quickly compared to the quadriceps muscle. Organized rehab is advised as a way to prevent this prolonged weakness.
An early intervention program may be able to improve the joint stability and prevent reinjury. Amateur athletes like yourself may be able to return to preinjury level of sports activity sooner after participating in a structured rehab program.