A couples massage involves two people getting massages in the same room. So if you ask for a couples massage, you'll be in there together, each with your own therapist. You'll be asked to remove your clothing and get on the tables under the sheets while the therapists leave the room. Underwear is optional. However, the less area it covers, the better. I hate it when guys wear their boxers on my table because it cuts me off from accessing half their leg. And I hate it when women wear those high-waisted granny panties because they cover half their lower back and I can't reach their SI joints, or their hips.
You will be under a sheet, so it's not like you're going to be totally exposed, but some people are really squeamish about it and rather than put them in a situation where they're uncomfortable and therefore tense, I'd rather just let them keep it on in the interest of getting them to relax.
For a full body massage, you'll need at least an hour. In thirty minutes, you'd get your upper body - back, neck, and shoulders only. It's really worth it to go for the full hour, because your muscles get tense in other places besides your back, and it gives you more time to be comfortable.