It's different for everyone... A person needs to demonstrate quad control, have full extension and have at least 90+ knee flexion before trying to walk without crutches.
Typically, (on average) at the 2 week mark post ACL knee surgery your Physical Therapist will have you begin to walk without crutches while still wearing your post-op brace (in the unlocked position). At this point, you'll begin to walk without crutches at home and also start to transition to one crutch (or cane) while walking outside your home. However, your knee will still feel a bit unstable at this point, so it's likely that at times you'll still want to use 2 crutches for getting around while making the transition to walking without crutches.