I had a lot of lining issues and then turned out I had Asherman's so it was all a moot point anyway. But the last RE I had told me to stay on the estradiol for a whole month before transfer, said that around 30 days is the maximum that a lining can be maintained and good quality enough to support implantation. Those extra couple of weeks really made a big difference and I did get pregnant for the first time ever with DE, but miscarried because it was a bad embryo. Moved on to GS for a lot of reasons, but just wanted to tell you my experience with building lining and with taking a long time to do it. I also had tried viagra, which helped a lot too, but it was those extra weeks that did the trick.
So you can go up to 30 days before you need to shed the lining and start over.