Sandbox is a concept and a virtual place where Google place the new sites to check their authenticity. This is done to keep a tab on spamming. There is no time limit for a site being in sandbox. The site can be in sandbox for a week to 6 months. some people claim of being out in 24 hrs!!!
You can break the Sand Box rules by submitting your website into Social Book Marking sites like Digg, Stumbleupon, Delicious etc
yeah it can be a myth but the truth is that every new site can't get the rankings instantly even if the keywords it is targeting has no competition at all. even if it does, the fluctuations in the rankings can be seen. moreover, Matt Cutts has confirmed that sandbox do exists but for some selected industry types and my experience tells me that it consists of the sites of commercial nature where spamming is done to generate leads and sales. i have done an experiment with a site that has nothing to do with commercial transaction or leads as it is a site of a NGO working for social cause. i never saw any sandbox effect on it. rather, it was and is ranking for the targeted keywords since last 6 months and has not moved an inch since then.
There is a complete segment dedicated to Sandbox... OP would have done well to access it directly.....The same is located here... Sandbox Regards, RightMan
The so-called "sandbox" theory suggests that new sites will be added to the Google index and may even show up for obscure searches such as the company/web site name but will not show well for other phrases that are relevant to the site. It doesn't matter if the site is optimized for the search engines either. In fact, those who have optimized their sites can drive themselves crazy making change after change but to no positive avail.
I must agree with An_Igma. Maybe there are some sandboxes in such crowded industries like MMO but in any other industries I had no problems at all.