Is there any minimum and maximum allowed time-span for acquiring a PhD degree?
What limitations a given student will face presumably depends on the policy of the institution in question, and in some places possibly national regulation.

At US universities, the graduate program often has mandatory courses, making it hard to finish very fast unless some requirement are waived. On the other end of the spectrum, many institutions limit funding after a certain period to try to push stragglers over the line (of finishing or quitting...), or at least make the PhD candidate jump through some hoops (e.g. petitions). Certain institutions have a limit on how long a student can be registered, but it's not clear to me whether this would block someone from submitting a thesis after finishing it on their own.

However, let's try proof by example. Here are some very short PhD durations (I think it will be much harder to finish this quickly nowadays):


Stephen Wolfram of Mathematica fame earned his Ph.D. from Caltech in a year
Robert Wagoner got his B.S., M.S., and Ph.D. degrees from Ohio State University in 1974, 1975, and 1976, respectively.


I'm less willing to list regular people who took a long time getting their PhDs, but I personally know some in the 10+ years range. A couple of more extreme example are


Theodore Streleski who ended up murdering his advisor during his 19th year. I don't think he ever received a PhD, but he was still pursuing it at the time.
Ingeborg Rapoport probably holds the record for the technically longest doctorate. She submitted her dissertation in 1938, but was stopped from defending it by the Nazi regime. Eventually, she was awarded her degree in 2015.


Obviously most people aren't going to be like these examples, so let's turn to statistics. In the US, NSF publish reports on e.g. time to degree of doctorate recipients. In education the median is 11.7 years. Presumably there will be some that took e.g. the 16 years you mention.