Strictly speaking the SNP would require Westminster to approve the referendum and agree terms otherwise the end result would either be the UK ignoring the result or cutting them off on day 1.
Realistically though, i don't think a referendum will be called in this parliament because i don't think the SNP believe they can keep momentum if they lose a second time and no polling surge has been seen. It's also worth saying that i do believe the No campaign did themselves damage by being so negative so if lessons are learnt then it's possible we could win bigger than before.