I've completely changed my approach to everyday wear since discovering merino. For everyday wear I have 4 undershirts total – a white SW microweight v-neck, a grey SW microweight v-neck, a grey SW microweight singlet (think wife beater), and a grey SW midweight crew.
Normally I'll wear one of the v-necks as an undershirt. Since I'm at a desk job, I can wear one of these for an entire week with no problem. Usually I'll wear each one for about 3 days total before washing though. I also have the singlet for under certain shirts where I don't want a visible undershirt and I have the midweight crew to wear under button-ups for sweater-type warmth on cold days.
I also have two microweight t-shirts to wear to the gym. I just wear it for the 1-2 hours I'm working out and then hang it up to air out when I get home. I can easily wear one for 5-6 workouts (probably even more) before washing it, whereas with cotton or synthetic shirts I'd have to wash a shirt after only one workout. Same thing with athletic socks for the gym. I have a four pairs of ultralight wool socks to workout in and just let them air when I get home, wearing each about 3 times before it gets washed.
Finally, living in Minnesota, I wear long underwear almost daily this time of year. I have two pairs that I'll wear on a daily basis – a microweight and a midweight, depending on how cold it is. Worn over normal boxer briefs, I would be fine going weeks without washing these, but I typically will throw them in whenever I'm doing a load of laundry anyways.
Basically all of this has allowed me to go from a drawer full of 8-10 undershirts, 3-4 singlets, 4-5 long underwear tops, 4-5 long underwear bottoms, 8-10 pairs of athletic socks, and 8-10 workout shirts to the 12 clothing items I described above. I do laundry about as frequently as I did before, but instead of two large loads it's usually just one small one.
I still wash underwear daily since sweat accumulation isn't exactly the only concern with those, but I've toyed with the idea of wool boxer briefs as well.