Got curious about the actual distribution so ran some queries on vndb [1].

Looks like the median renpy novel (with 100+ votes) takes 250min to complete with the ludicrously long ones [2] bringing the average up to 503min.

    engine      vn count  avg     25%     50%     75%  
    Ren'Py      251       503     120     251     571  
    KiriKiri    187       813     241     522     1143 
    Unity       65        871     224     452     1109 

[1] https://query.vndb.org/?sql=SELECT+%0D%0A++++r.engine%2C+%0D...

[2] https://query.vndb.org/?sql=SELECT%0D%0Avn.title%2C%0D%0AROU...

    title                       length (min)        engine
    HEAVEN BURNS RED            12580               CRIware
    Rance X -Kessen-            11085               AliceSoft System4.X
    Shoujo Settai               10418           
    Kamidori Alchemy Meister    8407                
    Seinarukana...              6858                
    Lessons in Love             6819                Ren'Py
    Sengoku † Koihime...        6744                CatSystem2
    Higurashi no Naku Koro ni   6626