Goldin 20 years.

People seem to enthusiastically support such ideas because they cannot see
any other options: go to mars or do nothing. I consider this the major problem we High Frontier types face when advocating our positions to other space enthusiasts. They ask ignorant questions before
I can say the bitat in 'Space Habitat' my favorite being 'but wouldn't it
just be easier to terraform another planet'. I've run into this same curious reversal of logic myself. One person I debated on a space newsgroup expressed the opinion that the O'Neill plan simply represented a lot of "unnecessary moving around of materials and wasting of energy, when you could instead just tweak a planet until it's just right." I expressed the opinion that "tweaking a planet" was a feat worthy of the Magratheans. People seem to feel that a preexisting planet somehow meets you halfway to your goal in a way that I don't feel is logically supportable. What it boils downs to is which is more difficult: building the equivalent of a small South Seas island from below the ground up, or totally reengineering the atmosphere and surface of an entire planet? I know which is going to be easier, but many don't see it that way. Call me ignorant, but I don't
see how planets that are naturally hostile to life (supposedly) could ever
maintain a stable ecosystem before crashing every 500 years! One of the arguments I got was that space habitats will require ongoing maintenance, while planets will not. I allowed that that was certainly the case for Earth, but like you, doubted that a terraformed planet would be maintenance-free.