Hmmh, personally I'd like to see them persist. It's not directly comparable to keeping and being able to use your med applicator as an other class IMO.
CE is a part of defense and I don't somehow like the idea that to have CE, the engineer is stuck being engineer forever.
Naturally a max amount of mines like in PS as well as perhaps a... few hour timer before they vanish if it's technically required? I know in PS fights can take all day sometimes, so two hours can be a short time, but that's more like "IF" it's required to keep server performance up, otherwise we might have thousands of mines around that wont ever see an enemy soldier
