How long do I have to wear tissue expanders after my last fill?

I usually wait 4 weeks if possible before performing an implant exchange. In theory, this give the tissue time to consolidate before the next stage of surgery. 

...I wait at least 6-8 weeks and preferably closer to the 8 weeks.  I will say that the longer the better.  I've had some patients come back 6 months after for one reason or another (marathon training, etc), and the breast just is easier and better to work with at the 2nd stage if you wait.  Of course there is a point of diminishing returns as well.