You're probably OK - keep a phone close I guess, if you start nodding uncontrollably, dial 911. Maybe ask a friend to come over and watch over you. Since you've taken 350mg before and not felt much, I have to think that 500 won't do much either. Oral opiate overdoses don't instantly manifest, they take awhile to build. You aren't going to drop dead; if you actually OD'd, you'd go into super nod dream land and stop breathing at some point.
Codeine kind of has a maximum dose ceiling. Codeine itself is psychoactive, but not very potent. Your liver metabolizes it to morphine, which of course is a lot more potent. I think that due to metabolism, the liver is only capable of handling 400mg of codeine or so. Someone else can correct me if I'm wrong.
So, due to both that and codeine's high histamine response, it's hard to actually OD on. You're more likely to puke first vs nodding out and ceasing to breath.
