* https://home-affairs.ec.europa.eu/policies/migration-and-asy... * https://en.wikipedia.org/wiki/Blue_Card_(European_Union)
A green card is probably equivalent to a permanent residence permit, those vary quite a bit between countries, and skills. Some countries might give a permanent residence in as few as 3 years, I believe quite a few have the 4 years threshold. Depending on what skills you have or how much you invest in the country you are moving to this timeline also shortens.
Rules have been changed now, citizenship in 8 years will become law on June 6th, also requiring language and cultural tests which weren't required before.
Continental Europe used to vary, Germany was stricter with 8 years to citizenship but permanent residence would vary depending on work skill and language skills.
Visas and residence permits are, of course, easier.