Hi I have poison oak rashes under my kneecap and around that area. I was just wondering when you know it is starting to heal.

You left out too muc. Where do you live? Are you sure of the cause of this? "Poison Oak" is in western US. The rash is an allergic reaction. It would be unusual for it to be confined to the area described and unusual to be the same on both sides. Eczema is common behind the knees. The duration depends on the diagnosis, 4 weeks after contact with a plant, much longer if eczema. Both respond to prescription cortisones.
Crusting. When the blistering is crusting over and there are no new areas of blisters occurring then the poison oak is resolving. Typically the course of poison oak is about two weeks, sometimes longer.