First of all, my friend, do you realize just how sweaty wearing a gas mask is ... especially if you're moving around or being active?
There are two variants of the 4a1 gas mask, one with a filter on the front and one with a filter on the side. Both are removable by simply unscrewing the filter.
You COULD, if you wish, leave the filter in place and NO HARM will come to you. They are, after all, designed to be worn for extended periods of time ... usually by military or police forces.
You are HIGHLY UNLIKELY to "expire or be used up" ... unless you already suffer with a compromised breathing problem ... and especially if you're not running about.