Do you know what kind of serviceberry you have? Some will get to be 25 to 30 feet tall or more. Spraying a mature serviceberry of that size would be an extreme challenge, to say the least. Some websites recommend spraying at bud break while others say to begin when the flower buds first emerge. They all appear to agree that multiple fungicide applications at 7-10 day intervals are necessary for control.

The rust does not significantly affect the health of the tree as long as the tree is not being routinely defoliated by it. My advice would be to either learn to live with it and accept that some years will be worse than others or remove the tree and plant something else that is less susceptible to disease.

Cedar-Apple rust has two phases ... on the serviceberry is half of it; control works best if you can treat both hosts. Of course, that's easiest if both are on your property ... if only the serviceberry is yours, it'll be difficult to resolve.

We actually treat the juniper more than we do much with the serviceberry. It's an imperfect treatment, since the juniper is 50' tall, and we only used a 6' ladder to cut off galls, but we definitely have seen a marked improvement between a couple years ago when it got introduced by a sick serviceberry and this year (almost not ugly orange blobs).

I imagine there are other hosts in the area, so yes, eradication is still unlikely. Oddly, the flowering quince only a couple yards from the serviceberry has shown little or no sign of infection, so no idea what's up with that ... perhaps it's not susceptible to the strain of rust we've got. I can't say I've seen the birds turn down serviceberry fruits even woth the rust ... they still clear out the tree ... maybe that's also dependent on which rust one is dealing with?

Anyways, back to the original question ... apparently the answer is starting as new growth begins to appear, and repeating at appropriate intervals ...

https://mortonarb.org/plant-and-protect/tree-plant-care/plant-care-resources/cedar-apple-rust/#management

https://extension.psu.edu/cedar-apple-and-related-rusts-on-ornamentals