I think 10 (+/- 2) years is a fair estimate but I would start counting from when Starship reaches orbit. It would be close enough to their historical developmen

I think 10 (+/- 2) years is a fair estimate but I would start counting from when Starship reaches orbit. It would be close enough to their historical development timeline of the Falcon which went from Falcon 1 reaching orbit in 2008 to a crewed Falcon 9 mission to the ISS in 2020, with a propulsive landing of the first stage to boot. I would expect a number of missions within that timeframe that could possibly satisfy the condition of "fly to Mars and back" but I think they will be able to do a full surface of Earth to surface of Mars and back to surface of Earth crewed mission at the end of that period. Call me an optimist but I'm pretty sure that's what they're aiming for.