Google Sandbox has two different meaning.
First, a period of time before a newly launched website is properly indexed and ranked. Google deliberately holds back indexing new sites before they trust it. If the site code is clean, has good unique content, and linked from one or two good sites, it will be indexed within a couple of weeks.
Second, a sudden drop of SERPs. This can last for several months, because google consider your website as too aggressive. Maybe you have lots of article and lots of links pointing at you in a short time.
Too avoid Google Sandbox, just be natural when you make your website, do not be too aggressive, and check your code for broken links.
