A new site gets quick traffic from Google, and after some time, the traffic drops down. the traffic stays down for quite some time, until the site has some good backlinks, good content. Sandbox effect usually happens to the sites with duplicates content.
In terms of rankings, a newly reistered domain may be said to be "in the sandbox" for the period it takes to start reaching the ranking level it should be at in accordance to its on page optimization, links, content etc. this can bearound 6-12 months depending on the theme etc.
It depends on how much work you do on the site, what it's niche is and therefore how popular it is. If you work hard, it's very possible in a couple of weeks.
is there any method to check if our site is in sandbox?? during sandbox does our site pages are indexed in google??