That 30 day period is pretty common. I'll add a bit of a twist to it though. It is possible thay a chicken can develop immunity to a disease, especially when they are young. Since they develop that immunity, no matter how long you keep them quarantined they will never show that disease but they can still pass that disease to your flock. For that matter, your existing flock may be immune to something but they could infect newcomers.
What I suggest is you pick a potentially sacrificial member of your existing flock to put with any newcomers and seee what happens. I don't know any way to get any safer.
I don't know what your risk is of a newcomer bringing in a disease. There are just too many unknowns. Many people bring home chickens from chicken swaps and all that and seem to never have a problem, but occasionally people's flocks get wiped out because of this.