Terraforming Mars by melting the frozen CO2 we know is stored at the poles, mainly the South Pole, is a much more difficult enterprise than some people imagine and I doubt it will ever be done.
If by miraculous means you could restore the Martian atmosphere to what it was 4 billion years ago, it might last a couple of billion years.
You don't stipulate any particular length of time you want it to last, and a billion years is beyond most people's comprehension. They talk earnestly about how humans could avoid destruction when the Sun expands to its red giant phase in about five billion years time, not realising that man will have gone to join the dinosaurs in far less than one billion, so two billon is an almost unimaginably long time.
But the fact of the matter is that Mars is a very hostile place, and we will have to put up with all the difficulty and inconvenience of its thin atmosphere and bitter temperature for as long as we are there.
To look on the bright side, things won't be so bad once we have established a large heated and pressurised base there, but to go outside the base you will always need a pressure suit and space helmet.