1 year is what MFG's say basically.
I replace mine about once a year, even if I don't play a whole lot. The cost of admission is what, 20 dollars? The cost of a lens is similar. The cost of my eye is much much much higher.
Besides that, in a year small scratches accumulate to make visibility less than optimal and I am the type of person that really hates wearing goggles / mask or anything on my face. I do what I can to make the experience as comfortable as possible.
The flex is just as safe as any mask except for the small foam ear covers. They expose your ears way more than they should. The side and profile of the mask is more protective than some masks like an I4 so, don't worry about that. The only part of your face that can really easily take serious injury are your eyes and your ears. Protect them..everything else is basically cosmetic compared.