This ride 270 mile ride took me 5 hours to go up and back by myself which mean reasonable speeds but not un-necessary stopping. For a group ride, I'd plan this for 6 hours since everyone will want to stretch and BS a bit. You can make this on one tank of gas each direction (it was 136.5 miles from my house to the gas station in Stanley). Boise to Idaho City: 43 miles of fairly smooth, winding corners that are a lot of fun if traffic is low. Idaho City to Lowman: 31 miles of twisting mountain summit crossing bliss. Lowman to Stanley: 59 miles of scenic mountain sweepers without much traffic and with a very nice view.