I had a real odd situation once where I was cleaning my rims, and the rag got caught on a little metal piece on the caliper and pulled it onto the rotar hence the squeaking noise. Sometimes it's the smaller of the many problems. After going to buy new break pads... my local mechanic showed me and bent it back... looked at me with a big grin and said "That should do it."
ALL NISSAN SQUEEL! i think usually brake pads last for about 30k miles until replacement is needed. thats when i did mine atleast. the rears were a bitch cuz of those stupid metal bracket things that holds em in.
the life of your brake pads depends on how you drive, and the quality of pads. if you brake too hard, or if you like goin 70mph and then brakin to 0mph very fast, and other conditions. brake pads can last 30K-40K miles if you drive like a granny. in my case they last from 7K to 20K.
CAUTION dont get those VGX Metalic pads or some shit like that for Summit Racing Equipment. cause all the shit they say about long life and low dust is BULLSHIT. i had them they last for like 5K mi. and my rims where dirty like a mofo all the time. right now i have PepBoys Premium (somethin like that) they brake realy good, and NO DUST.