So this happened... I earned the 90's milestone cape on Sunday. I would have thought it deserved an in-game notification. Now that I think about it I don't recall one for the 80's cape either so I guess not. It took me about 18 months to go from 80's to 90's. It will probably be another 18-24 months for the max cape. There will certainly be notifications then.
For those that don't know, my goal is to earn all 27 skillcapes (Invention will surely be in-game by then) on the same day. Maybe I can get it down to a couple of hours if I plan well.