How long does it take for an opening on the T-incision to heal?

This depends on whether you have already experience some degree of wound opening/dehiscence. If you have, it will obviously take additional weeks to fully close. In general, scars take a year to mature so they are always red, raised, and bumpy initially. Best to consult with your operating surgeon if you have specific concerns about your scar. Good luck!

Typically a small area, about half the size of your little fingernail,  can heal in 1-3 weeks but there is no definite amount of time. There are too many factors such as the size of the wound, the depth of the wound, whether or not it is infected, the amount of swelling and drainage from the wound to name a few. Additionally, there are other factors such as other medical problems like diabetes and whether or not your smoking and your activity level.
Your best option is see your plastic surgeon and follow their guidance and recommendations to ensure the quickest and optimal healing for you.
This area often times does have a little delayed healing and does often heal excellent.
Best of luck