Great question. It's been a long transition from flashcards. I developed the picture technique myself over a year and a half ago. If I learned a fact a year ago, if I studied it using the picture technique, I have about 70% recall if I reviewed it once 6 months ago. If I used flashcards for a month, I have 5% recall now. If I used flashcards continuously (~1-2 hrs per day) for 6 months then stopped using the deck 6 months ago, I have about 10% recall.

(I did a very cursory self-test, then approximated these numbers. It's very far from perfect, but I didn't pull them from a hat. For the memory palace numbers, this is pulled from what I'm currently studying, so is reliable.)

I am still in the honeymoon phase for memory palaces. If I reviewed them once a week after creating them, I have about 90% recall one week following that. I currently have 100% recall for the 4 lists ~10 items long that I made last month and have reviewed 3 times each, but haven't reviewed in the last week.