I am coming from the perspective of someone who never had to wear glasses at all most of my life, then suddenly getting into my mid 40s I started having some problems reading things at distance (was a big problem while driving). My first pair of glasses were progressives. The point of all the above is: I didn't already have a lifetime of training my brain to wear non progressive glasses, so maybe that makes some difference to what I say next.

For me, the first several months were beyond difficult - I was convinced many times that the glasses were defective and almost gave up on them several times. That period included rediculous awkward head movements to find perfect combinations of head and eye angles to read things at different distances with lots of micro stuttering movements to dial it in. But, like magic, and just like the optometrist said, it finally did click in my brain and the act of wearing and using them became second nature. Two years later, my prescription had changed enough that I needed new lenses and frames and I basically had to repeat the entire process of adjusting. I've done that three times now and it doesn't ever seem to get any easier for me, but I always eventually finally adjust until it becomes second nature for me. At this point, I have started to conclude it's not a process of learning as much as it is a process of my brain literally rewiring some of my internal vision wetware and that process must take about 4 months (for me).