I've read that one of the reasons Mars doesn't (any longer) have an atmosphere is that it loses more gas to the solar wind than the Earth does, because its core has solidified and it therefore lacks a magnetosphere.
It seems unlikely that we could re-liquify Mars's core, or in any other way re-create its magnetosphere.
But we could theoretically supply Mars with an atmosphere through some combination of bombarding it with icy asteroids and possibly nuclear weapons.
So if we managed to give Mars a shirtsleeve atmosphere/environment, how long would it last before we needed to top it off with (carefully placed) further asteroids?