The Google sandbox concept is that Google keeps websites from ranking well really fast.
That’s a vague definition because Google has never confirmed that the sandbox exists.
The theory is that Google wanted to prevent spammers from creating a site, quickly blasting it with backlinks, and ranking it quickly before it’s penalized.
The sandbox would make that spammer business model less desirable. So, it actually makes sense.
My advice is to start your site as soon as possible, start working hard by publishing great content, then get some backlinks and promote your site. The more work you do, the better. The 6 months will go by fast and then you can stop worrying about it.
I’ve seen sites get way more traffic because of higher rankings after 6 months. The sites typically have decent rankings and get some traffic before 6 months, but Google seems to push sites that are under 6 months down to the bottom of page 1, or more likely page 2 or beyond.
By the way, my belief is that the 6 months starts after you start actively working on the site, i.e. publishing content, promoting the site, and get backlinks.
Often the traffic climbs dramatically after 6 months go by, and it’s almost to the exact day. There are theories about what you can do to get out of the sandbox. For example, if you use the keyword golden ratio and target very low search volumes, there’s a great chance you’ll get significant traffic under 6 months.