Let's say your domain has been registered for more than 2 years, and its optimized for a certain keyword, but not even showing up in the top 100 SERP, is there a way to get your site out of the sand box?
Has there been content for your keywords on that domain for 2 years? Or are you just saying the domain is 2 years old?
Google sandbox is nothing physical but a place where websites reside after they have created some illegal ways of promotion.
To get out of that box you cannot do anything much as you can find that reason of sandboxing, deleting those links or whatever it is and then submiting request to Google for reconsidering your website.
What's the usual time sites stay in the Sandbox? 6mos? 12mos?
There is no such specific timing. If you can find the reason it was banned and remove them then its easy for you to submit your website back to search them for recosideration. But its still not sure how much time Google takes to get back your website in search engines.
The Google Sandbox is an alleged filter placed on new websites. The result is a site does not receive good rankings for its most important keywords and keyword phrases. Even with good content, abundant incoming links and strong Google PageRank, a site is still adversely affected by the Sandbox effect. The Sandbox acts as a de facto probation for sites, possibly to discourage spam sites from rising quickly, getting banned, and repeating the process.
The sandbox is basically a filter that made by Google to evade spamming of search results by unscrupulous webmasters. Before sandbox was made, new sites could rank fast, which permit "black hat" SEO professionals to make quick buck by ranking new site on top of Google's index for viable keyword, and then cashing out before Google caught up with their spam method. These domains usually were registered only for short period, exploited some loophole in Google algorithm and depend on sudden bursts of traffic over their limited life span.
There is no such thing as sandbox. If you can't see you website in google search, just wait for a few days. I have already experienced this. I thought they punished my website but I was wrong, it now has PR 1.