When will the pocket of fluid in my lower back go away? (photos)

Thank you for the picture.  You need a drain placed to treat your seroma.   You also should call the surgeon that performed your procedure even though he/she is not local for guidance.

You likely have a seroma, although it is difficult to tell.  It could just be swelling.  Without examining you and knowing if surgery was done in that area, it is hard to give you better advice.  If it is a seroma, it could respond to drainage and compression.  Please let us know if the area had liposuction or not.

is not something that I would consider a common occurrence unless you had more procedures done than being mentioned.  Regardless. if free fluid that can be aspirated, it should be done and you will have to find a local surgeon to manage that for you.  Fees will apply.  If its just swelling, it will resolve with time and compression.  If your surgeon is from the US, you should be contacting them for further advice.

When you say your surgeon was not local, do you mean he/she was out of state or out of the country.  If they were out of the country and this was done to save money, I would suggest going back there for follow up.  That's one of the problems they don't tell you about when you go for surgery.  In the USA most of us would pick up the phone and call a friend and explain the situation.  I would say that Dr. Blinski is being very generous with his fee as most would charge 4-5 times that amount for taking over your care.  I'm sorry you are having this problem but as they say, sometimes discounted fees can be very expensive.

After reviewing your long RealSelf file I question the judgement of going to the DR for additional surgery. Now you have a recurrent seroma than needs insertion of a closed drainage system. Since you already used Dr C. Mendieta, why not return and pay for the minor drain insertion surgery with him? Or is he refusing to see you? Than next option is fly back to DR for this surgery. Or if I were to offer care my fee would be a global $2,000 for operative re insertion of coccyx drain and all the follow up care involved. It seems the cheaper way will now become quite expensive. Sorry for ALL you issues. 

Swelling is common, but a distinct fluid collection of the back after a tummy tuck and liposuction is unusual. You should contact your plastic surgeon, and he/she should be able to give you the name of a good plastic surgeon locally. You need to be seen.