How long does optic neuritis last when there is no underlying cause? is two weeks after diagnoses too late to start steroids?

Optic neuritis: Optic neuritis can leave permanent visual defects. Depending on the cuase of optic neuritis. Optic neuritis may improve quicker with steroid use. If optic neuritis is due to multiple sclerosis, then it has been shown that if you use steroids on the initial episode it prolongs the time of onset for other symptoms of multiple sclerosis.

Get treated: There is ALWAYS a cause for optic neuritis, and it should be treated. Best approach is to use intravenous methylprednisolone, or intramuscular ACTHAR. But if not treated, your outcome is likely permanent loss of vision. See ophthalmologist or neurologist and get this addressed ASAP