Bringing the Appalachian Trail gateway to Mt. Cheaha would add a little more than 300 miles to the current 2,190 miles of the AT, bringing the total to around 2,504.
This would include 92 miles from the Cheaha trailhead to the Georgia State Line, then 169.2 miles from the Georgia line to the Benton Mackaye Trail, and then 71.4 miles to Springer Mountain, GA, the current southern gateway of the AT
Extending the AT would add slightly less than 3 weeks to the normal six month trek (using the 15 mile/day average of AT hikers). Given that Alabama would allow for thru hikers to begin hiking comfortably in February or March, it still allows time for the full journey to Katahdin before the Maine winter sets in.