Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Sudden vision loss, a curtain-like shadow, many new flashes or floaters, severe eye pain, or vision symptoms with signs of stroke require emergency assessment rather than a routine appointment.
And for anyone, someone who:
If home monitoring has been recommended, ask what change should prompt a call.
Do not wait for the next routine appointment if vision worsens rapidly.
Contact the eye clinic promptly for a new but gradual change, increasing trouble with daily activities, or new treatment side effects.
Follow-up depends on the cause and whether vision is stable.
A clinician may repeat visual acuity, eye pressure, retinal imaging, or visual field testing.
Keep scheduled visits even when there are no new symptoms, because some eye diseases progress quietly.
Routine follow-up is appropriate for stable, gradual problems being monitored by an eye care professional.
The article also says
Bring an up-to-date medicine list and report any changes in sight, treatment side effects, falls, or difficulty completing daily tasks.
Every sign above is quoted from the sofidex article on Blindness and vision loss. If you are not sure, err upward — it is always reasonable to be checked.