Replacing a valve cover on a bmw e36 4 cylinder is a rather quick process, typically taking about 1 - 2 hours. The worst part about this job is if the valve cover gets stuck to the head, in that case it can take some serious hits with a rubber mallet and prying to get the valve cover to release.
Typically the valve cover gasket is replaced when you notice that it is leaking oil or you'r changing the spark plugs and happen to notice that one or multiple plugs are filled with oil. 
Overtime valve cover gaskets become hard and brittle, they eventually become susceptible to oil leaks.