Are you working with a technical team?
I suspect you’ll have to experiment to gain knowledge, and apply it to your unique situations.
I’m not here to give you absolute guarantees.
How far do your radio links work? How do the local weather and atmospheric conditions vary a different distances.
I’d say at approaching 30 km the value of the common view tends to be significantly diminished.
For post-processing the sweet-spot is around about 7 km, that’s not to say 10-20 km can’t be servicable, but you’re likely to be disappointed more often.
Perhaps look to the experiences of surveyors in your own area, and the density of stations they are using, and the use of VRS to create more localized service from more broadly dispersed stations.
You can also increase the density of your own stations, or increase the sophistication of your implementation.
https://wisconsindot.gov/Pages/doing-bus/eng-consultants/cnslt-rsrces/tools/wiscors/default.aspx