What is the maximum time an expander can remain in place before it has to be replaced with the implant?

Typically tissue expanders are replaced with implants at 4-6 months.  But there is no rush to have them exchanged.  The worst likely scenario is that it ruptures, the saline gets absorbed by your body, and the expansion goes away.  Dr Rodger Shortt

There is no set time limit on how long the tissue expander can stay in place.  They are frequently left in place 1 to 2 years When a patient requires radiation postoperatively. This being said the devices are not built to last as long as a regular breast implants, so they will have a higher rate of rupture in the 3 to 5 year range. 

Tissue expanders can remain in place for years without any problems in most cases. Since they are saline-filled devices eventually they will fail and deflate. But when that will occur can not be predicted.