There’s no set timeline for a venous ulcer. This type of wound can be slow to heal.
The general markers for wounds are about progress rather than completion. You may have a non-healing wound if it has not started to heal in 2 weeks, or has not completely healed in 6 weeks.
Started to heal, and completely healed, are two different tests, and the two-week one is the more useful of the pair. A wound can be taking months and still be healing. A wound that has not begun to change in two weeks is a different observation.
For wounds that reach a wound care center, the picture is plainer still: non-healing wounds may take months to heal, surgery may be performed to speed up healing, and some wounds never heal completely.
What shortens it is not a secret, and it is the same as the treatment. Wear compression stockings or bandages every day as instructed. Put your feet above your heart as often as possible. Take a walk or exercise every day. Take medicines as directed. And if ulcers do not heal well, your provider may recommend certain procedures or surgery to improve blood flow through your veins.
The measurements taken at each appointment are what answer this question for your own leg. A wound being measured is a wound being tracked against itself, and that is a better guide than any general range.