As cool and monumental as it might be to see them hit their 20th studio album, I could see them calling it a day after two more albums, or maybe even one more honestly. If they write and record next year, and then tour into 2025, that'll be their 40th Anniversary, which is the perfect time for another retrospective tour to close out their career. If they're feeling like they could do one more after that, probably releasing in 2027 and then tour on that for a year or two, they could maaaaaaybe make it to 2030 for their 45th Anniversary, but by then, I doubt James will be in touring condition for most of their material. He'll be 67 by that point and if he's been having trouble now in his late 50s into early 60s, it'll probably not get much better. I know JP could probably write, record, and tour until he can no longer play, and I'm sure he will do that beyond DT's end, but I think the band would be best served to bow out gracefully and know when it hang it up.
TL;DR - Worst case scenario, they do one more EPIC album and tour, then do a 40th Anniversary Tour to call it a day in 2025. BEST case scenario, they do two or three more albums and go until 2030, but I doubt they'll last beyond that in their current line-up.
As for Jordan and JP, I could see them wanting to do more LTE after their contractually obligated fourth album. I wouldn't mind a couple more of those albums between now and 2030, and let's see an LTE Tour somewhere in the next few years too!
-Marc.