.....or if you're building an kernel rpm for distribution / compiling one from a source rpm, it also depends on your machine specs.
FYI - On a 1.3Ghz Duron w/ 512MB, a vanilla compile from a source rpm (using set defaults taken from the old config file) usually takes about 20 - 40 mins. It will take longer if you watch it
Compiling from source (i.e. a tar.gz) takes longer - 1 to 1 1/2 hrs, but you'll probably spend more time deciding about what to include / compile as module / leave out.