Hours of play per level is a better guide than sessions or weeks/months to level, yup. 8-12 hours to level is pretty typical; my 5e online game might be 15 hours to level on average, but text-chat is much slower than tabletop. My Pathfinder game is typically about 8 hours to level (ca 2.5 sessions), my 4e game 12 hours/4 sessions. My Classic game probably closer to the 4e rate.
Edit - my 5e game: 35 online sessions, around 3x35=105 hours or so of play, PCs are 8th level or
nearly 8th, so 7 level-ups. That'd be exactly 15 hours of play to level, though it was faster levels
1-4 and slower now. If it were tabletop I'd expect around 8 hours/level.
Edit 2 - checked logs:
5e: 35 sessions, 105 hours, 7 level ups. 15 hrs/level.
4e: 92 sessions, 276 hours, 24 level ups. 11.5 hrs/level.
PF: 33 sessions, 115 hours, 12 level ups. 9.5 hrs/level.