How many years of overall experience in technology should someone have to become a CTO in SMEs and Startups? 

It is a combination of years + results + innovation + business knowledge to become a CTO.

I selected 5+ years, but it is far more complicated than that. I have known people with a couple of years of experience who made great CTOs. I have known people with 20+ years of (often very senior) experience who could never be a CTO (especially in a startup).

The most important thing to me is someone who has learned to seek out and accept help and guidance is those areas where they are not strong. No one can be an expert in everything.

I was surprised that the highest percentage is at 4-5 years. To gain enough relevant experience in that amount of time to manage software engineering teams, DevOps, R&D, etc along with the appropriate people management skills, maturity, and communications skills to work effectively with other company departments seems unlikely.