The "no" vote in the late '70s (in the Devolution referendum) meant a twenty year wait for another vote (in the late '90s) - basically a generation later.
Similarly, I'd guess that the "no" vote in the AV referendum a couple of years ago has basically ruined any scope for electoral reform (at Westminster elections) for a generation.
The vote on EEC membership in the '70s meant that there was no appetite for another vote on Europe any time soon, as far as the majority of the population were concerned.
(I say all this without getting into any debate about whether any of these results were good or bad - there are more than enough arguments about the rights and wrongs of Scottish Independence/ the EU etc on the Internet - no need for another here)
So, if Scotland votes "No" later this year, then I think it'd be hard to find enthusiasm for another vote any time soon. But I suppose that this depends on the closeness of the vote. If there's a 51% "No" vote in a few months time then I could see some pro-Independence people encouraged to try to get another vote (though obviously if 51% of people vote "Yes" there'll be no chance of a referendum on "Should Scotland rejoin the UK").
TBH, I'd say that if I were in Salmond's position I'd make sure that I could be certain of the answer that the people would give before I asked the question. Asking and being rebuffed will hurt the SNP's cause. However, Salmond is obviously a lot cannier than Clegg (who rushed into the AV referendum without building any case/momentum for reform) - I'm wondering whether he's timed this quite nicely to give himself the best chance - years of SNP administration at Holyrood, ensuring that he's called a vote whilst the unpopular (to most Scots) Cameron/Osbourne Government are in Westminster (i.e. more of a "them and us" than if Labour get in in 2015). Like Boris Johnson, he knows what he's doing (and knows that sometimes it helps if you give people the impression that you don't, so that they underestimate you).