How long do you have to wait for AI after Lupron?

I am pre-menopausal and was hoping to use tamoxifen to treat my bone mets. Unfortunately, after my latest PET scan, my oncologist has stated the tamoxifen isn't working and that I need to be moved onto an AI. He told me to stop taking the tamoxifen and within two days my period resumed and now my bones feel bruised and I'm thinking that even if the tamoxifen wasn't stopping the spread it may have been slowing it?? Anyhow, the next plan is for letrozole and palbociclib. However, since I am not in menopause, I need to get Lupron shots. My oncologist told me that the lupron would reduce my estrogen levels within days. Upon some research, I see that some women aren't in full menopause for a few months after starting lupron and begin taking AI after their estrogen levels test at an appropriately low level. Does this mean that I won't be able to start my next course of treatment for potentially months??? Or if my oncologist starts me on the new regimen immediately, does this mean that I will just be experiencing the side effects of the drugs without any real benefit until the lupron successfully puts me in menopause months later? Any clarity on this would really help!! Anyone else switch from tamoxifen to an AI while being pre-menopausal? What was your process? Thanks.