From this graph it can be seen that Water Vapour can never be held by the Martian gravity, and would escape into space.
How would future terraforming processes overcome this loss?
We've discussed this before with respect to the moon, which is even smaller.
Hanging on to an atmosphere for thousands of years is just fine for human activities. As it leaks away, the residents will replace it.
Jim2B can tell you how long it would last on Mars if such replenishment stopped. But, with all the infrastructure in place to put the air there in the first place, it makes sense to just plan on leaving in on at a slow drip to keep the air at the desired level.