
No later than a month old as by then they are screaming to have more space.
Pups are covered by maternal antibodies to all the normally encountered bugs Mum is exposed to, and as your Garden should be private the risk of serious infectious disease not brought in by Mum or kennel mates is negligible even with visiting wildlife.
We make such todo about taking pups out before vaccinations yet don't do anything like this with human infant who are not fully vaccinated for around five or 6 months.
Like Isabel I don't vaccinate my pups before they leave me, unless they are stayiong past 10 weeks of age.