It kind of depends on your budget and the ground conditions. Water jetting under a concrete walk can take only minutes, or it might take hours. Are there any trees in the vicinity? This is not the kind of job that lends itself to a serviceman who has to charge by the hour. Pros don't have to water jet, since they have special boring equipment, but the use of that equipment requires setup and excavation beyond what simple water jetting involves.
By the way, this water jetting is only for concrete sidewalks. No blacktop, and no pavers.