It depends on the discipline, the programme, and (particularly) the country. I did mine in the UK, where the funding is for 3 years and the expectation is ≤4. I managed that (modulo two terms of sick and paternity leave) and so do most others in my field. The quickest I saw was 2 years 6 months, by someone with an impossible combination of intelligence and relentless 9-hour productive days. Some take longer (financially and professionally problematic), not that many drop out.

In some places in the US taking >8 years is normal. In some parts of Europe it's an actual job, with delineated teaching responsibilities, a pension scheme and everything. In Russia the equivalent isn't even called a PhD. It's not a standardised process.