I think the process varies across every county (and between all Jury Commissioners). Where I live I have no problem calling the Jury Commissioner on the phone and explaining that I have commitments and she will pull up her list of trial start dates and go through them with me to agree an probable date that I think I can commit to. Sometimes it is three weeks away, sometimes three months. And it seems that as long as I let her roll my name forward to another date I can postpone many times.

But this is in a county which sees very few, and usually short trials. Though since it also has a small population, the summonses come around with boring predictability.