I ordered my vents from hexvents.com, they're a vendor here as well. If you've got a coupon code from phastek you might come out ahead.
I don't keep my guards in, only when i wash the car...as for how much dirt gets in the bay, I really can't tell ya. I keep her in the garage all week, so my car stays pretty clean overall anyway.
Time required...IIRC, it took me about 4 hrs, I think? I took my time, and a liberal amount of breaks as well, so your time may vary. The instructions provided with them are very well-written and easy to follow if you watch the install video on their site beforehand.
Build thread linked in the pic...