The airplane can maintain FL410 until it runs out of fuel.
The limitation is the people on board. With a pressure-breathing oxygen mask, a typical pilot will likely remain conscious for 15-20 minutes or more. However, if said pilot has never experienced a rapid depressurization, and doesn't know what to expect, he may well hyperventilate and/or panic. The pressure changes on the body are NOT trivial, and each person reacts differently, both physically and psychologically.
OTOH, with only the dixie-cup O2 masks in the back, any passengers and cabin crew would be unconscious in a minute or less, and dead within a few minutes. There is not enough ambient oxygen above 35,000' to sustain life.