I personally dont think its that big of a deal. I havent got down to science of things but I figured since noone else has put any input in I will.
I would think by this by and age, (edit: reputable) cold air intakes do I pretty good job at keeping dirt out. As good as the stock oem ones? Maybe, maybe not. But Ive never heard first hand of (reputable) cold air intakes hurting the longevity of someones motor. I also think at least 90-95% of the hot air (if you mean caused by the engine heat) can be dealt with if the intake is properly shielded.