No, you do not need a new N95 mask after [#] hours.

Particulate respirators get MORE EFFICIENT (they trap more particles) as you use them. From what I've been able to piece together from technical reports and government studies, there’s no danger in continuing to use them as for as long as you can physically breath through them. So instead of stockpiling enough to throw a good mask away every day, give your extras to people who are outside without one, especially homeless people.