The European Movement, Scotland's oldest dedicated pro-European grouping, said in report for MSPs that it was unlikely that an independent Scotland's application would be blocked.
However, it warned political will would be "paramount," particularly in talks to secure a share of the UK's valuable budget rebate.
The report, submitted as part of a Holyrood inquiry into an independent Scotland's membership of the EU, added: "The 18 month timetable is vulnerable to delaying tactics and the political context in which negotiation would take place will be highly dynamic.
"If this period is extended, this could cause disruption, more so in the case of a new application.
"A potential counter measure to avoid hiatus and improve Scotland's position while negotiating is to leave open the date of formal separation."
First Minister Alex Salmond has signalled his intention to declare independence on March 24, 2016, ahead of the next Scottish Parliament elections.
He has argued the timetable would allow sufficient time for talks to divide up the UK's assets and debts and join international bodies such as the EU.
Scottish Liberal Democrat MEP George Lyon said: "As part of the UK, Scotland benefits at the EU from things like a share of the big UK budget rebate, key opt-outs on the euro and control over our borders.
"People would not want to see this special treatment frittered away as the cost of securing a quick settlement in Brussels in the event of a Yes vote.
"The European movement has no particular referendum axe to grind and they are simply restating what many experts have already told us: that the SNP's independence timetable in the event of a vote to leave the UK is not realistic."