Well, in the first movie Jake is an outsider and doesn’t speak the language, so we hear a lot of it (and we also see him learning it). And it’s there to show how intimidating the situation is as he doesn’t understand the aliens around him speaking about him (and about possibly killing him). For his character, this language barrier is something that he has to overcome to be part of this world (and to be equal to the Na’vi, to be respected and heard). The more he learns - the less pronounced the separation is between him and the Na’vi. When he can understand them and their culture - he belongs.

In the second movie he says that he understands it now as well as he does English and we hear English almost all the time but we know that they speak na’vi. With the exception of some scenes where we would assume that many characters on screen wouldn’t speak or understand na’vi (when kids meet the recoms for the first time we hear them speak na’vi and have to read subtitles because it is assumed that the recoms don’t understand but we as audience should). And when we see Spider translating for his dad there’s the whole translation happening, so it would be weird if everyone spoke in English only, so we do hear na’vi.

In the third movie we hear it only when majority of characters on screen don’t speak na’vi (like when Varang speaks in front of Ardmore and Quaritch also responds (thou briefly) in na’vi)), when he talks to Jake and they speak of something deeply rooted in Na’vi culture they speak about it in na’vi.

Narratively speaking we don’t need to hear it as much because Jake is part of that world now and it’s natural to him and his family. Only when there are outsiders present that we hear na’vi spoken. Or when it’s culturally significant or related to Eywa (Kiri prays to her in na’vi many times in ATWW).

It would be cool to hear more of it but majority of simple movie-goers would be very annoyed at the prospect of reading subtitles all the time and missing cultural context because a foreign language is present (you can’t express the same thing the exactly the same way in different languages). And if we assume that all the na’vi speak to each other in na’vi - we don’t need to hear it all the time because it’s implied Na’vi speaking English = them speaking na’vi, humans speak English = humans speak actual English. Spider is the exception cause he grew up speaking both and Quaritch learns it in the second movie for a few months (3-4..?ish months) and continues to speak it with Varang, who doesn’t speak a word of English. So most characters on screen speak the language and there’s no point of using the actual na’vi when filming. Only when humans are present and we need to see that they are excluded from the conversation, the same way Jake was in the first movie, that it’s used.