Mild cases of thrush do eventually go away on their own without any medical treatment and if the thrush is a mild case of thrush then it should go away on it's own within 14 to 30 days.
Sometimes it can take as long as a few months for the thrush to go away but eventually it will.
More severe cases of thrush may not go away on it's own and may come back and get worse.
If the thrush doesn't go away after a few months or the thrush seems to get worse then it would be best to see a doctor about it for treatment.
If the thrush is more severe and the thrush is left untreated then it can spread to other parts of the body.
I would recommend seeing your doctor or go to a walk in clinic to have the thrush checked out and possibly treated so that it goes away sooner and doesn't have the potential to spread to other parts of the body.