"Once in a generation" should mean around 20 years, no? This seems like a reasonable time span. It would be a huge mistake for the independence camp to rush into a new referendum too soon (unless some dramatic event significantly and durably alters public opinion, but it's hard to imagine), it would probably backfire.
One event that could produce calls for another Scottish independence referendum would be if the UK votes to leave the EU in 2017/2018.
The Scots are generally pro-EU with the English being anti on average.
The irony here is that Alex Salmond spent years and years preaching about an independent Scotland within the EU and using the Euro as it's currency... until the financial crisis struck in 2008.