40 years. Same as the gap between the two Scottish independence votes, and the 40 years for the chance to vote out of the EU. Come back again in 40 years for another vote (OK, if you're picky, and very pissed off with one or more of them, somewhere in the high 30s)
Sturgeon and the SNP would accept a referendum any time. As I said, for seismic events like referenda, 40 years is about right (the difference between the two Europe votes and the last two Scottish independence votes (on which they've voted 'no' twice already).
So, 2054 it is. Let it be so.
You are of course assuming something you don't know - whether the EU would admit them. Apart from their dodgy economy if not joined to England, the EU might not be too keen on a land border with England.
The Scots should have thought it through before they sent their king down south to take over England and Wales too, then. :)
Referendum after referendum until you get the 'right' result isn't an appropriate way of doing things. Forty years between referendums seems about right - they should be...