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 now if there is:
And for anyone, someone who:
Sudden vision loss, severe eye pain, or inability to open the eye needs urgent assessment.
Contact a poison centre or seek prompt medical advice if a child has chewed the plant, the plant cannot be identified, or symptoms are more than mild.
Medical assessment is also appropriate for repeated vomiting, worsening pain, persistent voice changes, or difficulty drinking enough fluid.
The article also says
Continue rinsing an exposed eye while arranging help.
After eye exposure, seek advice if pain, redness, tearing, light sensitivity, or blurred vision continues after thorough rinsing.
A small exposure with mild symptoms that are clearly improving may be managed at home after poison-centre advice.
Monitor for increasing swelling or swallowing problems rather than relying only on the amount of plant material involved.
Every sign above is quoted from the sofidex article on Dieffenbachia poisoning. If you are not sure, err upward — it is always reasonable to be checked.