As EP suspected my estimate was off significantly. However, hardly in the direction he suspected.
http://www.users.globalnet.co.uk/~mfogg/gjerde.htm
"A consideration of the stability of the new condition of Mars is of course interesting. McKay examined this. He found that Mars would approach the present cold and dry conditions that Mars currently possesses, but the process would take approximately 100 million years. This degeneration is fully acceptable, and requires only a very modest maintenance to avoid."
Unfortunately the basis for this estimate is not detailed at the link above. Japan's Nazomi should help answer this question more clearly as it is designed to measure how fast the solar wind is currentlt stripping off Mars atmosphere.
Regards,
Mark
Terraforming Duration