Most of my missions took about a week to complete. I try to complete all the tasks for each flight day in real time, then just let to sim run, or save and come back later to start the next days flight tasks. To date, STS-125 took by far the most time due to the 5 loooong EVA's, and other real life events that kept me away from the computer for several days at a time. I think it took me over three weeks to get STS-125 home again.