Spark plugs

Slightly of topic but still spark plug relayed ... how long does your spark plugs last??? Obviously depends on a lot of factors. I'm running 315whp at 0.8bar boost and run prefix as well. I use bur9eqp all round.
 
And what are symptoms of bad plugs? When I drive hard mostly in second and third under load it feels like a vibration from the engine. Not sure if it's a misfire. I'll check plugs this weekend. Just wondering what typical symptoms are. And thanks for the replies :)
 
I run bur9eqp all around and make considerably more that 450bhp. 450 is my low boost setting ( 7 psi ). I change plugs typically once a year. I do take them out regularly and clean them though, about once a month. I also run AEM start coils in direct fire ignition setup and water meth injection.