How long can I wait with flashes of light to see if it goes away without seeing a doctor?

Dont wait. Dont wait any longer, can be a sign of serious and irreversible eye problems, the sooner you go the better are chances to fix the problem.
See a doctor. Flashes of light, especially those accompanied by decreased vision, need to be evaluated immediately. There is a possibility that the retina is experiencing a problem (hole, tear or even detachment). If there is a detachment, you usually have no more than a 72 hour window for treating it to prevent severe vision loss. Do not delay seeing a doctor - go immediately.
See EyeMD ASAP. Flashes of light are a potientially serious symptom so you should not delay in getting medical attention.