What age range are you thinking of? I've seen middle grade & YA graphic novels that are as short as 80 pages. ZebraFish by FableVision/Peter Reynolds seemed short for the target age & it's 128 pages (not too short, just on the shorter side of norm). Sugar Falls: A Residential School Story by David Robertson & Scott Henderson is 40 pages long & shelved in the middle grade section at my local library - I see one recommendation online for ages 13 - 17 probably because of the difficult subject matter.
John Martz's Burt's Way Home is 48 pages. I think it's an all ages comic but seemed quite sophisticated - not really a picture book age book. Felt more suitable for ages 8 & up.
Ashley Spires has a number of GNs (Binky, Fluffy, Gordon) for ages 7 - 10, they range from 64 to 72 pages.
Jennifer & Matt Holm have a series called My First Comics - they are 22 page board books.
There are also plenty of PB that are heavily inspired by comics (Arree Chung's Ninja comes to mind, there are many more).
I think there is lots of leeway with graphic novel length. It would depend on the subject matter & how many pages you need to have to properly tell the story. In multiples of 8 of course.