I usually get 12-15 hours total out of IGAs games so I don't see why this - being 25% larger than the older games - won't top that. For me, that is. I don't rush through things.
I got more than a hundred hours for each of the GBA/DS IGAvanias. I don't stop until I have everything, all the drops in the bestiary, everything maxed, ect.
In Aria, for example, I maxed out all the items, since the max cap for everything is a single digit. I also had maxed gold and maxed level. Maxing gold in Aria of Sorrow, while having maxed soul eater rings, isn't the easiest thing to do, but I was younger and my parents didn't buy me more games.
I feel like I should be expecting to play Bloodstained for a veeeeeeery long time. I see some people talk about the game only lasting 10 - 15 hours, and I wonder if they're planning on beating it once, then tossing it.
Personally, if IGA says the game will be large, I'm thinking it's going to be colossal.