OK, some quick numbers.
1,392 7.5 minute maps cover Washington State.
3/4 of those would be 1,044.
I looked at some of the "Cache Restricted zones" like National Parks, Indian Reservations, Hanford works, National Wildlife Refuges, National Recreation Areas, Military Bases, Wilderness areas. Several of these areas have several 7.5 minute quads completely inside of them. I quit counting when I got to 100 quads that can't have caches, and I know there are a lot more. As much as I'd like to explore areas of the Pasayten Wilderness some day, visiting the 24 or more quads in that wilderness to find or place caches (and technically caches aren't allowed in Wilderness Areas), just won't happen in my lifetime. And I'm sure there are some quads where I've found the only cache in that quad. Heck I think there are some DeLorme Gazetteer pages that I've done all the caches in. Doing the DeLorme Challenge, County Challenge, and the Lookout Challenge covered a lot of ground.
I love maps, and I love challenge caches, but this is too much for me.
I say downsize this challenge to the 100K series maps. That would be about 57 maps I think. That would be a little more fun. And it would still take you on a tour of the state.