Google Sandbox : A belief
Google sandbox is a filter to prevent the new websites to check them whether they are spam or not , or we can say it is a filter which checks the new website is as relevant as the older one or no. Because they are not as popular(relevant) as the older sites are. So sometimes, web content developer feels exhausted as their site can not be at the top on SERP of google as the sandbox tries to verify the newer site.
This observation of google sandbox depends on the quantity and quality of keywords of your site have. It may take 6 to 8 months to keep your site in sandbox.