About how long will a dielectric union last before the brass and copper eat each other up and leak?

Dielectric union will last for ever have not heard any one leaking, Thanks.

John

Installing A Dielectric Union To Join Galvanized And Copper Pipes - Plumbing System Components

I agree with John, the brass and copper do not touch and are separated by a plastic collar. I have never had one fail, yet. Are you tansitioning from galvanized to copper or what?

Yes it galvanized to copper on a hot water heater