How long do you leave your block heater plugged in?

[KZF250]

quick question about replacing the block heater.

I have a brand new one, my old one works but I got the engien out soo..migth as well replace it.

Any kind of sealant needed or jsu unbolt the old one, bolt new one in and bobs your uncle?

I never have changed one but seems I read somewhere that all you should need is some coolant on the o-ring...maybe someone else will chime in.