how often does an average metal song change key? im usually write folk type songs and if i want to change keys i'll just go to the relative major for the chorus or bridge . But in a metal song i want to stay in a minor key so im finding it difficult to decide when and how to change key. thanks for your answers.
Songs arent supposed to be written logically, imo. There is no set formula. You should change the key when it feels right, not because you have to.
A type of key change I find quite common (and imo works great) in more melodic metal is changing to the subdominant's key, primarily by going to the subdominant's VI then taking it from there, but also in other ways.

Dunno if that made any sense (feel free to correct me if my wording is off), but here's an example:

Em - Em - C - Am
Em - Em - C - D

and into the subdominant's key

F - F - Am - Am
F - F - Am - Am

Here's how it sounds (I transposed it into Em, can't remember what key it goes in the original and I don't have a guitar/perfect ear with me :p):


Key change at 1:41

Though anything goes in music obviously!