They are capped weekly dude. And from the news of it, it doesn't look like it takes much to actually gain Renown.
Look here:
https://www.wowhead.com/news=316846....we-know-so-far
>> There will be a weekly cap on how much Renown you can gain.
>> Renown is earned through playing the Covenant Campaign and by completing two weekly quests.
Playing the Campaign makes sense (and who doesn't want to play the Campaign) and completing 2 quests doesn't say much, but it's definitely not something that should take a long time.
As such, it doesn't seem like there's much grinding involved to keep up your Renown.
And if you don't want to... then don't? xD
P.S. There is also a catchup for alts, so it seems like good design at this part.
Also: If you are behind, or you switch Covenants, or you are playing an alt, there is a catch-up system already in place. << this here shows that even if you switch the catchup is activated on your main. I got to give props to Blizzard for this design at least.