I've been here (on the forum) a while and I can't say I've noticed a pattern for controller launches.
The original KL88 which I still own was launched in 2015 while the MkII came four years later in 2019.
Assuming the 4-year cycle was is typical, it looks like it might be late next year when the MkIII arrives, with MkIII 49's and 61's arriving before it.
Though all my words are just 100% supposition because the people who do know won't tell us.
Compared to some other companies (e.g. Apple, whom we know launch a new iPhone, i(Pad)OS and MacOS every Autumn, reliable as clockwork), Arturia are very tight-lipped about hardware, software and firmware launches.
IMO if I was in need of an 88 key midi controller RN and I was satisfied the KL88 MkII met my needs, I'd just buy one. I mean let's be honest, it's only a MIDI controller: a MkIII, whenever it's launched, isn't exactly going to be bringing features with it that cure Covid, end wars and eliminate world hunger; it's just going to be more of the same wrapped-up in a slightly different package. You can spend your whole life letting FOMO spoil your enjoyment of the 'now'.