I find it unpredictable. I remember the last band I was in some songs came to me quickly, some took awhile. Some of it depended on how well I could sing the part. If I had it nailed then my brain knew it and it was fairly easy to learn as long as I had the technical chops for it.
These days I mainly play fiddle and Irish Tenor Banjo and I'm learning a bunch of tunes (working on learning about 150 new tunes). I find that a lot of the learning revolves around patterns. If a tune has patterns that are ingrained in my head then the learning (not necessarily memorizing) is fairly easy. On the other hand if a tune has a bunch of patterns that are unfamiliar or that I don't have ingrained in my brain yet then it can take a long time to learn the tune. To help me, I write out patterns that I'm having issues with and practice them every day without working on the tune they're in. The processes of writting them down helps me learn them and going over them without working on the rest of the tune gives me the reps I need for me to learn them.
These days I mainly play fiddle and Irish Tenor Banjo and I'm learning a bunch of tunes (working on learning about 150 new tunes). I find that a lot of the learning revolves around patterns. If a tune has patterns that are ingrained in my head then the learning (not necessarily memorizing) is fairly easy. On the other hand if a tune has a bunch of patterns that are unfamiliar or that I don't have ingrained in my brain yet then it can take a long time to learn the tune. To help me, I write out patterns that I'm having issues with and practice them every day without working on the tune they're in. The processes of writting them down helps me learn them and going over them without working on the rest of the tune gives me the reps I need for me to learn them.