How long should you wait before finishing a new basement?

Basements are almost always built wrong, and that is the main reason that they turn moldy. This is a fault with building code, where it often forces builders to finish basements with vapor barriers in the worst possible place, right behind the drywall. So as long as you build it correctly you should be okay.

It does take years for the concrete in basement foundations to fully dry, but that is assuming they have been protected from moisture infiltration, which again rarely ever seems to happen. So I would go ahead and finish it as it is likely as dry as it ever will be, and this article spells out the science behind it –

Why basements are moldy, and how to prevent mold when finishing basements

The most important point to realize is that backfilled basement walls cannot dry to the exterior as the moisture will only encounter wet dirt, so the only place moisture can go is the interior. And if you put a poly vapor barrier on the wall AFTER the studs and insulation, you are sealing organic materials in with wet concrete, which is the perfect environment for mold to form.

So – put your vapor protection against the concrete first then the stud wall, that’s how you keep your wall dry. Any moisture in the wood will harmlessly escape into the basement air where it can be easily managed by ventilation systems and dehumidifiers.

Here is another page that will help you, there are techical drawings showing the best ways to insulate basement exteriors, interiors, or both. 

Best practices for insulating basements to avoid mold 

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