I can usually get a piece into playable condition within 2-3 months. Then after another 4 months or so I start to feel that it is really learned and ready to perform. Now that I'm back in lessons my excellent teacher often finds things to improve for an additional couple of months after I feel that something is ready. I tend to work on a few pieces at a time. Right now I have more working pieces than usual (six) and several are multi-movement but some are not very difficult note-wise so the learning time is about the same.
I do have the best performing experiences with pieces that are deeply internalized from multiple years of playing, and the pieces I love most are the ones that still have new things to discover even after multiple years. Hey, I can now say "decades" about some pieces I still play
There are exceptions; I do learn much easier pieces much more quickly, if I want to play something new for church one week for example. But then even though the piece is easy I often feel like I'm only scratching the surface. Pieces that are very long or very difficult either technically or artistically can take me much longer than 6 months.