In the US anyways, you are probably best off consulting the program first, rather than the supervisor. PhD programs will typically advertise some statistics on time to completion, like mean/median completion date, and possibly "% complete by ____ years". I would look for this information first.
In most cases PhD durations in the US are indeed open-ended, in contrast to what your question suggests; unless otherwise specified, there will be no strict duration either for an individual advisor or program. There may be upper limits, but most students will strive to graduate well within these limits.
If you do ask your prospective supervisor, I'd start by asking about their past students rather than a prediction (or promise) for you. As mentioned by others and in the comments, PhDs are not about putting a set number of years in and walking away with a degree, they're about taking the necessary time to grow as a researcher and write a suitable thesis. Different people complete the process at different rates. Time limits tend to come as funding runs out and although they can sometimes accelerate process towards a degree (and convince a committee/advisor to help you expedite things a bit) they also represent the point of failure, the date at which you no longer have support to continue and must leave without a PhD. This scenario is not really in anyone's interest: the advisor, the student, the committee, the graduate program all want you to succeed.