I don't know how his retirement will work in terms of his contract with the UFC. IIRC, he signed a new multi-fight deal with them just recently, so there's probably non-compete clauses in there that would prevent him from fighting elsewhere once he retired from UFC (as long as there are still fights on his contract).
All this to say - I do think he'll retire from UFC pretty soon. I also think he'll return to competition (in UFC or elsewhere) not too long after that. The Ngannou fight is always there for him, and with the recent influx of Saudi money into the PFL, he may be willing to wait 3-5 years and then show up to collect that paycheck there.