How long do you let a chalazion go before you visit an ophthalmologist?

As long as you wish: Chalazia, in and of themselves, don't harm the eye, though if they are big enough they can either obstruct or blur the vision. Hot compresses, 10 mins, 4x/day for 3 weeks will often get hese to resolve, but if they don't and you want it gone, that is the time to see the ophthalmologist.

3-4 weeks: We usually recommend 3-4 weeks of waiting and conservative treatment with warm compresses. Most will resolve without need for intervention.