When should I inform telnet about my salary increase?

I'm finally making more and I want to actually be able to bear the fruits of my labor :( . Idk how much more my monthly payment will go up. When should I tell them? I'd rather just get it over with. Since SAVE was canceled ( I wasn't paying anything on that), I was advised to go on an IDR and have been on that for a few months. I'm paying off the interest only monthly. I was advised by two different people on nelnet to do switch to IDR and that paying the interest monthly will still count towards my payments yet keep my interest from accruing.

But I was also told that the people who work for nelnet also don't know much. I'm sure they get some brief training and that's about it, some of them don't even have advice.