To answer your quesiton truthfully, it takes between two and three weeks for immunity to form.
In ALL my years of having dogs, training dogs and dealing with pups and adult dogs, I have never seen validity to this huge panic to <<<<<SOCIALIZE>>>>> before some imaginary window snaps shut.
It won't. Simply be upbeat, fun and confident -- "Oh, let's go see THIS!" ... and if you think it's fun, approach it as fun, and continually expose this pup to many, many, many new things it's whole life long it won't fear them.
I've taken fearful fully grown dogs and have socialized them with exactly that way with never a problem. If you are comfortable, confident and not panicked the dog won't be either. Life is an adventure -- LIVE IT UP!!
But do it safely -- my dogs travel all over the place -- we take them everywhere and they do pet therapy in many many different places. but I frankly find dog parks too dangerous -- you might think my dogs were horribly treated but we just plain don't go. Around here they are too much the place of fights and disease.
It all depends on what you're personally comfortable with.