I use primarily Ilford FP4, HP5 and to some extent Agfa APX 100 (it's SO darn sharp!!). I haven't used selenium as a negative intensifier for decades but have a few recent LF negs I think could benefit from it.
Kodak Rapid Selenium Toner 1:3 will get you about one stop's worth of expansion, if that's what you're after. Times will be around 4-10 minutes, in my experience.
Make sure it's been thoroughly washed, and pre-soaked it a little while before you put it in the selenium. Just don't leave your negatives in there too long, or go do something else and forget they're there (for example). Probably the easiest and best intensifier I've used.
(I've tried this with FP4+ and APX 100 with good results, from negatives originally developed in both D-76 and pyrocatechin).
Selenium intensification is always proportional as to the density of the print or negative. So therefore, by definition and by result it will have very little effect in areas of low density, ie. shadows and greatest effect in areas of greatest density, ie. highlights. That is why it will expand negative contrast range by approximately one zone as others have related.