I’m content with a couple of 20 milers, but I have gone up to 22. Usually, when I’m bumping up my long runs I do it in 15-20 minute increments. So I’ll run 2 hours one week, 2:15-2:20 two weeks later, etc. Since my body is used to that 15-20 increase, I figure my longest run needs to be within 15-20 minutes of my goal time. Example, if I’m shooting for 3 hours, I’d like to run 2:40-2:45 for a long run. Since I run them fairly slow, 8:00 to 8:30 pace, that’s right around 20 miles.
As for timing, I’d say at least 2 months out, if I want to get in at least three 20 milers. Figuring backwards, my last one would be 3 weeks out, then 5 and 7. This is just a general guideline. I’ve done more than three 20 milers prior to marathons and I’ve done 20+ milers 3 out of 4 weeks too. I believe Joan Benoit did them every 10 days. That’s kind of hard with a “regular” job.