The length of time it takes for your rash to heal really depends on your body's immune system. Some people can clear up in days, some take up to a month.  A few important things to note:

If your rash isn't spreading, then you shouldn't have to keep washing with Tecnu. If your rash is spreading, that means you are coming in contact with the plant's urushiol oil again. Usually recontamination (if you haven't been around the plants again) happens because you are picking it up off an object that has been in contact with the plant oil such as gloves, gardening tools, mountain bike, pet's fur, etc.. Those items may need to be washed as well.

If the rash isn't spreading but it is getting worse, you may want to consult a physician.  Signs of infection can include very red skin, puss, swelling, fever and pain.

If the rash isn't getting worse, it may just be that your body needs a little time to heal.