I don't even know what a nukige is, but almost every rpgm games i played last over 20 hours each (well, i like playing them slowly, exploring and completing 100%)
Anyway, i'm playing this now and saw that they aren't rpg, just a visual novel/point&click/puzzle so of course they are short (but still fun for me xD)
Basically what most ppl call hentai games when talking bout japanese porn games are in japan refered by two names, Eroge and Nukige, with the first ones being longer and more proper games with deeper stories and the erotic content usually being as an adult fanservice either used as events when following certain characters side stories, those games are usually made in a way that it is possible to release a non erotic version usually called 15+ version or all ages version. Eroge also are made into normal anime not hentai anime for example Grisaia series, Utawarerumono, Tears to Tiara.
Nukige on the other hand are basically fap games, short, simple with a story made just so its not a gallery of random images. There are some longer Nukige, and some with some decent stories to tell, but you should never expect too much when playing on of those.