After you get omicron ( fever three days only ) , how many months are you immune after you test negative again ?

Unfortunately, there is no absolute answer for this question because the Omicron variant keeps changing. You can be infected with the current Omicron variant BA-4 or 5 but over next month or so you can be infected with a new variant (please note that I do not use the term re-infection). If you are symptomatic, I would assume you are protected from re-infection (same variant) for at least 6 months.