It depends on the person, like how much time he or she has for practice, and how quickly he or she learns. It also depends on their learning style and their background. For example:
I started piano when I was 8 and had a summer of lessons. I stopped playing for 5 years, and came back not remembering anything but a few memorized treble clef notes. I tried to teach myself, but I ended up playing at about level 6 or 7 by ear, and only being able to read at about level 3. This summer I finally got back into taking lessons. Since ear was all I really knew, and that's how I naturally learn anyway, it's taking me a long, LONG time to learn to read instead of just listening to a piece to play it. If I had a different background or a different learning style, it could change the way I learn to read music- for better or for worse- and it's like that for everyone.