Depends on what you mean by complete.
https://completionism.com/events/legion-remix
Getting all of the items, achievements, feats of strength, and reaching 999 versatility took me about 40 days of semi-casual play. After completing the quest campaigns and getting all the dungeons completed on M+20, that mostly consisted of logging in, grabbing the infinite research quests, and doing some or all of the raids on mythic difficulty, and doing one M+49 for the feat of strength.
They've implemented catch-up mechanics and there are surely a lot of pumpers with 999 vers running around now, so the pace should be quite a bit quicker, but completing all the campaign quests will still take some time since you can only speed that up so much.
Leveling alts can be done in a matter of hours if you just want to get them to 80. If you want to fully complete their class order campaigns it'll take a bit longer.