I have been getting some interesting info recently regarding this.
A top SEO consultant has stated that it is common when link building to get sandboxed. The way that they gauge this is if they are not getting results even if they are searching for "websiteurl.com" or "companyname+location" and dont get on the first page for these terms (ones they were previously ranking for easily e.g. long tail). They stated it was a temporary effect and a side effect of link building.
From what I gather when sandboxed your site then receives some kind of review. I believe the review could potentially be a manual one which could explain the random timescales, we have also seen G recruit like crazy for manual quality testers previously? Also domains less than 6 months old are more vulnerable to this.
More and more my research points to aged domains being a very powerful factor indeed.
Also I have a friend that is tendering for an SEO (I was way too expensive for him, he is going mcdonalds prices lol) but in this process he spoke to one of the top SEO companies in Europe and he knows a little SEO and grilled them about their link building. They stated they aim for 15 links max per day in their link building activities.
In conclusion I think that the sandbox is bad news if a piece of sh1t site, autoblog etc. etc. or just temporary prob if you are white hat site with good content. Logically it can only be a really good thing if you pass this "test" IMHO.
Another tip I read here recently is to throw up sites like crazy then leave them to age a little bit before they start developing e.g. several months as many people have experienced working hard within 6 months and getting limited results.
From doing a ton of reading I am still not sure what a bulletproof amount of link building is but I believe it to be approx. 100 links per month for white hat sites. Another great tip was link building through services that approve your links to make look organic e.g. directories, commenting, emailing web masters etc. In theory you could go mental with directories and they will approve at different times anyways, some like DMOZ will take ages. Other advice I have gotten here over and over is to mix up the types of links e.g. directory, forum, comments, articles, videos, networking etc. etc.
I am not super experienced but hope this helps some ppl as I have been reading up like a fiend on some of this stuff.
