don't put it on fast mode, you want it on normal.
the difference between the 2 is that fast mode skips steps that are normally done, and as a result, i belive it means you'll only have direct lighting on the map, rather than having it bounce/reflect off of surfaces in a more realistic manner.
regarding compile time - it depends on manner factors including the map geometry (i.e. the size of the map, how much of the map is on 16x16x16 grid, did you use func_detail intelligently, etc), your CPU speed, number of CPU cores, RAM and the amount of other programs running in the background.
for lighting, you should also consider the amount of models and func_details as they need to be lit too. - and of course the number of light sources and the lightmaps on your brush faces.
for me, with a 2.0ghz dual core and 2 gig of ram, i usually need between 5-20 minutes to do a compile for a release version of a late beta stage map. - but i don't compile with HDR turned on (it bloats the file size but adds a feature that, though looks really nice, is rarely noticed in the thick of battle, and custom maps need to balance filesize with other trade offs to a certain extent).
if you are compiling with HDR turned on, then the vrad part will take double the amount of time as it has to run 2 times.