How long have you been in Plumbing business for/How long have been serving the Hollister area?
You'll always want to ask how long the company has been in business in Hollister. While this isn't a guarantee that the company will do good work, companies with longevity on their side must be doing something right. That being said, there are newer companies that do great work. If the company is under three years old, ask where the owner was employed before and for how long. If he/she has a long history of employment, it's reasonable to infer that he or she decided to go into business for themselves after so many years of experience.
What is your projected timeline for the Plumbing project in Hollister?