Brennan won't retire until his health breaks. He wanted to go on and on and on, and regretted being 'forced off' the court when he was IOTL. (He was 84!)
Marshall's health was pretty appalling, and he could conceivably have been forced to stand down, or even died, as early as under Carter IOTL. But again, he had a limpit-like attachment to remaining on the court - he stated publicly he wanted to die on the court, and won't be in any hurry to retire, particularly if there's a Democratic president in office. A two-term Democrat is going to make these two less likely to retire early, not more.
It's really not as simple as 'President of this or that party is in office, justice stands down around the time they did IOTL' as people like to assume in these threads. It's more complex than that. I think it's possible if a Democrat wins in 1980, they might end up filling Stewart's seat, but neither of Marshall or Brennan's.