IF...
IF 35,000 > 47,000 ft
4 minutes at 50 ft/s (a v. high thermal climb rate in the troposhere, 20 ft/s is quite normal)
If a/t out, manual flight, the throttles would need to be cut quick if encountering that sort of thermal bubble. And going 'over the falls' out the backside of that could be quite an experience.
IF...