Assuming that's correct about 5-10 years (though honestly I doubt it takes that long). The question is - why do you assume that after 10 years gas suddenly stop

Assuming that's correct about 5-10 years (though honestly I doubt it takes that long). The question is - why do you assume that after 10 years gas suddenly stops heating atmosphere? Gas will stay there for the practical purpose "forever", enough to heat atmosphere for thousands of years.