Of course it varies by person, but they generally take small naps with the goal being that the nap is short enough the boat doesn't travel "past the horizon" or in other words past where they could see when they were at the helm. That's speed dependent, but I've seen them say 20-40 minutes naps. Further, there are systems like AIS (automatic identification system) that broadcast your location that depending on the area most boats above a certain length will have on, so your receiver can be set to alarm if a beacon comes within a certain distance. You can also set up a radar system to alarm if it detects anything in your path in a certain distance, those are notoriously unreliable though. Plus you have a VHF radio that can be set to scan and someone might hail you in time to stop a collision. With those on, people who are willing to accept more risk will sometimes take longer sleeps and just risk it, especially in less congested waters. That channel in particular recently had a comment about accidentally sleeping through their alarm and going for several hours unattended.

But solo sailing longer passages is inherently a dangerous proposition.