I've already done a few 200k-s, and I plan to complete 300k next year. Also had plans for 400k as well, I suspect I'll be able to overcome it, it's just one sleepless night, with may be 1-2h of sleep along the route somewhere.
But starting 600k, it becomes a problem. Riding 2+ nights without proper sleep is too much strain for my body, and I wouldn't risk it.