Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Call emergency services for sudden major or complete vision loss, especially if it occurs with facial drooping, arm or leg weakness, trouble speaking, severe dizziness, or a sudden severe headache.
These can be signs of a stroke or another emergency.
Emergency assessment is also appropriate after a serious eye injury.
If a chemical enters the eye, begin rinsing it with plenty of clean, lukewarm water immediately and seek emergency advice.
And for anyone, someone who:
Arrange urgent, same-day assessment for:
Book a routine eye examination if blurred vision persists, keeps returning, or affects reading, work, driving, or other activities.
Gradual changes, frequent headaches with visual tasks, worsening night vision, or an outdated glasses prescription also deserve assessment.
People with diabetes or an existing eye condition should follow their planned eye-review schedule and report new changes rather than waiting for the next routine visit.
Every sign above is quoted from the sofidex article on Vision problems. If you are not sure, err upward — it is always reasonable to be checked.