How quickly would an atmospheleak? (was Mars)

--- In spacesettlers, DOUG MAY wrote:
> 2. Mars has no magnetic field like the Earth's since its
> core has long since frozen due to its small size. No
> magnetic field means no magnetosphere, which means Mars
> is unprotected from the solar winds. When Hydrogen and
> other light weight gases float up too high, the solar
> winds easily blow them away, leaving Mars with a very
> thin atmosphere.

http://en.wikipedia.org/wiki/Terraforming_of_Mars#Dealing_with_radiati
on

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Venus, however, shows that the lack of a magnetosphere does not
preclude an atmosphere.
[...]
In the past, Earth has regularly had periods where the magnetosphere
changed direction and collapsed for some time. Some scientists
believe that in the ionosphere a magnetic shielding was created
almost instantly after the magnetosphere collapsed, a principle that
applies to Venus as well and would also be the case in every other
planet or moon with a large enough atmosphere.
=====================

> No amount of terraforming will ever increase Mars'
> gravity or restore a planetary magnetic field,
> therefore any atmosphere "created" on Mars would
> leak out just like the old one did.

How quickly would it leak?
http://en.wikipedia.org/wiki/Terraforming_of_Mars#Background

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the thickness of the atmosphere and abundance of
water, [are] both considered to have been lost over
the course of hundreds of millions of years.
[...]
none of these processes are likely to be significant
over the typical lifespan of most animal species, or
even on the timescale of human civilization, and the
slow loss of atmosphere could possibly be
counteracted with ongoing low-level artificial
maintenance activities.
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