How long should a nut last assuming its a cheap plastic type one that would be on a 300 dollar guitar. I use nut lube but when i change my strings lately the nut slots are dirty like worn off metal and my tuning stability has been taking a small turn for the worse lately. I do a good amount of bending and my guitar is 6 months old (played every day). Could it be worn out and if it is would a brass nut last longer if i replaced it with one? How much might that cost to have done?
nut lube, hahaha, i lol'd...

the nut could last an incredibly long time, it all depends how you treat it.
you should be able to hit people in the face with your nut and it should be fine, a plastic nuts pretty cheap though