Hi,
Relief pitchers generally go rusty based on how much they actually pitched. A pitcher who pitched in 120 games would get rusty due to inactivity soon. The 30 gamer would get rusty less quickly. Also, rust can help some pitchers, it's not always a detriment. Some pitchers benefit from more rest. In the game it appears it's effects vary quite a bit depending on the player and his playing time.
Doc