The Herald’s Graham Avery thinks 3 to 4 years and carefully inserts the qualification ‘fully’. In January 2014, he said 18 months: https://www.bbc.co.uk/news/uk-scotland-scotland-politics-25965703
Fabian Zuleeg, chief executive of the European Policy Centre think tank in Brussels, in May 2021 said:
We are talking here about a part of the United Kingdom that, also because of Brexit, seeks independence and wants to belong to the EU value community,” Zuleeg said. “And in this respect, I think the mood on the EU side is rather positive. He suggested an independent Scotland might be able to conclude membership negotiations in two to three years, similar to Finland’s accession process in the mid-1990s.https://www.politico.eu/article/haste-ye-back-how-scotland-could-rejoin-the-european-union/
The average is just under 5 years: https://ukandeu.ac.uk/explainers/how-new-member-states-join-the-eu-all-you-need-to-know/
With regard to that ‘fully’, Finland joined in 1995 but did not join the Euro until 1999 and did not join the Schengen area (border-free) until 2001, six years after.