How long do gask mask filters last?

Question

I was curious as to how long a gas mask would last. Say if it is exposed a lot to whatever chemicals and **** and your walking through it with your mask on. How long is the filter going to last before it needs replacement, roughly?

Answer 1

Say there was a major earthquake and major natural gas lines are broken, natural gas would be spreading  at a fast pace, would it be advisable to have gas masks in my emergency kit just in case?

If so how often would my family and I need to change the filters? Also what type of mask would be best for children and adults? Thanks for any information you can give.

Answer 2

What about chemicals such as paraformaldehyde? I know pathologists eat with that stuff around, but I really don't want to get cancer down the road.