Persaonlly, I think that 40 hours is a little excessive, especially for a Ratchet title (though I think 40 hours worth could be suitable for a Jak & Daxter game), but ITN was way too short! I think that ToD was the perfect length for an R&C title, it had plenty of levels and plenty of other things to do. Within the parameters of the traditional R&C format, I think Tools of Destruction is about as good as the series could get in terms of length/pacing.
Into The Nexus was very good, it's gameplay some of the most enjoyable in the series (especially the museum level, which pays tribute to tons of stuff from throughout the franchise) but it lasted no time at all. The old games used to last me at least a week, if not more, just doing a single playthrough, but I finished my first playthrough of ItN in less than a day!
Though, like I said, if a R&C game was 40 hours long, it would either overstay it's welcome, or, like Darkstar said:
If they want to make a 40 hours + R&C game, they better add a TON of content, don't divide the sectors, make the radio as varied as GTA V, get around 15 different types of collectibles, make the Side-Quests and optional stuff 70% bigger than the main campaign, have some form of customization.
they'd have to change it up so much gameplay-wise that it might as well not be in the R&C series for the same reasons that people said earlier in this thread that A4O and FFA should have been part of a new IP…
Though I wouldn't say "no" to most of those additions anyway… having ACiT space exploration but with everything in one big sector? Yes please! And customisation is something I loved about the Constructo Weapons in ACiT, I'd spend ages tinkering away trying to find the perfect combination that I liked for my weapon, and then even more time customising the colour scheme to absolute perfection! I was slightly disappointed there wasn't something like that in ITN, even if it had just been changing the cosmetic design of the weapons…