Devils tower can be done in 4 hours, maybe a little longer if you hike up from the CG, Mt Rushmore it's self can also be done in 4 hours, however the rest of the area is a minimum of 2 days, if you only have 7 days between Yellowstone & Tetons, I would do 2 at the Tetons the rest at Yellowstone, in actuality neither is sufficient.
We did 10 days in Yellowstone, wish we did 14, This year we will do 3 at the Tetons.
Some folks say you can do Yellowstone in 2 days, perhaps if you just give it the old Chevy Chase head bob, but trust me, if you go through Yellowstone that fast, that's just what you are doing, just "going through" you're not seeing much.
