He managed to prolong his career by going to a nation that is not as competitive as the teams he played before, he used to play at teams like United, Real Madrid or Juventus, and looking at Al-Nassr right now, we can easily say that they are not a team that is as good as those clubs. However, even with a drop like this, we have to say that how lower could he get with age? I mean not like he will play in some third division Portuguese team or something, he will have to stop at some point.
At his age, many players retire or still continue as players but can not appear too regularly because of injuries.
First we have to admit an impression from Cristiano Ronaldo that he still has very strong body at his old age that helps him to have many appearances for Al-Nassr and his national team in this season. He will not have appearance for his teams if he gets many injuries.
Second we have to admit another impressive thing from Cristiano that he still is able to score many goals and I agree that he smartly moved to a lower level league to get both, more money and avoid too severe competitions, to prolong his career. Although, he did it for some reasons, his performances in Saudi Professional League are still very impressive. We can see that how many players who are younger than Cristiano, can score many goals like him in Saudi Professional League. This is fact that anyone dislikes Cristiano must see.
I believe that because of this, there is a potential that he will not be able to continue for a long time, maybe one more season but that is about all I can see from him, afterwards he will be way too old to continue playing at any level.
I don't know but if he wants to play, and, if his body allows him to play, he will continue his player career.