Skin ulcers should heal up on their own within a few weeks to a month or so depending on how bad the skin ulcer is.
Some skin ulcers may take as long as 12 to 14 weeks to fully heal up.
So while it may seem like the skin ulcer is not healing properly or at all most skin ulcers are slow to heal so you just need to be patient.
If the skin ulcer hasn't healed within 12 to 14 weeks then you might want to see your doctor about it but usually they heal up without medical treatment.
You can help the skin ulcer heal a bit faster by using a saline solution to clean the ulcer daily.