How long will it take for necrosis open wound to heal?

Hello and thanks for the question. Sorry you are undergoing this complication! 
Wound separation, delayed healing, and compromised tissue are unfortunately known complications of tummy tuck surgery. The risk of these complications are discussed during the consultation and pre-op appointments so the patient understands the possibility of these occurring. 
In my experience, usually wound care with dressing changes is enough to promote healing in these areas. It takes a few weeks to months to completely heal. Your case would depend on the extent of open wound, any risk factors you may have that could impair healing, and the development of secondary complications such as infection or large amount of drainage. 
The best recommendation is to continue following up with your plastic surgeon and follow the instructions and medical advice he or she gives you. 
Best wishes, 
Rohit Jaiswal, MD   Plastic Surgeon 
Las Vegas, NV and Lake Havasu City, AZ

Dear sagonzalezbonk, I'm sorry to hear about your problem. This is not uncommon after tummy tuck. It is hard to tell without an examination, but generally speaking, it can take weeks and even months in some cases for this type of wounds to heal. I would suggest you to keep your wound clean and follow your surgeon's instructions.Daniel Barrett, MD, MHA, MS 
Certified, American Board of Plastic Surgery  
Member, Am. Society of Plastic Surgery

There are far too many variables and relevant information that you have not provided that would be important in making a more helpful and somewhat accurate conjecture as to how long it would take for your wound to heal. If the wound is regularly cleaned and appropriately treated and dressed and your health and nutritional intake are good expect healing to take from one week to up to one to two months. Approximately!Steven Turkeltaub, MD