Hello,
I'm a complete beginner. I have half a dozen self seeded cedars (probably Mediterranean) from a now removed tree next door. Each one is in a 3" pot and they seem very happy. They are 3-4 inches tall. How long should I keep them in an ordinary plant pot before transferring to a bonsai container?  A couple have roots peeping out of the bottom of the pot so presumably I can wait until the autumn. I'min the UK

 Thank you in anticipation.
Pauline

You can keep them in nurcery containers until they have reached around 75% of the final trunk size. For even faster result, plant them in the ground for a few years.