I wash them when it's convenient. My 8-10 gym shirts will see more washes in their lifetimes than all my other shirts and pants combined.
Daily: socks and underwear; t-shirts if I've been at the gym in them
Every other day: t-shirts
Once or twice a week: pants, outer shirts (if worn often), sheets and pillow cases, bathroom towel, kitchen hand towel, dish towel.
As required: Jackets, shoes, hats, gloves