I really have no idea about the filter timer. But if you're breathing faster and heavier, it's probably time to switch out your air filter for another one... which will also clear up your vision. The best advice I can give when playing this game: don't bother with stealth, at least not on your first play through. This is one of those games where alerting one enemy instantly alerts every other enemy in the area to your exact location, so you may as well just go in shooting. Stealth can be a very fun way to play the game, but it's also VERY hard to pull off. The other best advice I can give: leave no stone unturned. It's a good idea to search every body and every little crevice off the beaten path so you can find extra ammo, and weapons. Finally... play this game alone at night in the dark. It is SUCH a thrill, trust me! Have fun in the Metro! ;)