Almost never.
A few years ago I read an opinion that E1M3 is very hard on nightmare, so I tried it, beat it within maybe 1hour, beat it again, and just moved on.
Sometimes I see a design which seems very inspired from original maps, so I load them up to check if I was right.
That was Doom and Doom II. I do play TNT at a ratio of maybe 1 map per 2 years and Plutonia, yes, more often, but still, it is maybe 3 maps per year, so in a decade I would play the whole thing, if only, some maps I don't touch at all :-P