TL;DR: Nine years.
The timeline of events relating to Bertha’s imprisonment is (approximately) as follows, relative to the year Y in which Jane arrives at Thornfield Hall:
Y−14 — Mr. Rochester marries Bertha Mason
Y−13 to Y−11 — Mr. Rochester’s brother dies
Y−10 — Mr. Rochester’s father dies
Y−9 — Mr. Rochester moves to Thornfield Hall, imprisons ...
Yes, a reader of Wide Sargasso Sea who is unaware of its connection to Jane Eyre is pretty much guaranteed to have missed the entire point of the novel.
Jane Eyre is so iconic that Rhys could simply assume knowledge of its plot while writing her own book. The heroine of Wide Sargasso Sea, her husband, and her half-brother are all very important characters ...
I have a few details to add to the existing answer.
Bertha had been in the attic for around 10 years. This can start to be worked out from the date given at Rochester and Jane's attempted marriage by Briggs who announces the impediment of a previous marriage:
I affirm and can prove that on the 20th of October A.D. (a date fifteen years back)
There is no consensus among critics about whether Wide Sargasso Sea stands on its own or not. For example, Francis Wyndham wrote in his introduction to the first edition (1966):
For many years, Jean Rhys has been haunted by the figure of the first Mrs Rochester—the mad wife in Jane Eyre. The present novel—completed at last after much revision and agonized ...
Confiant has the following entries under ‘chè’ (and nothing under ‘che’ or ‘ché’):
chè 1 cher, coûteux
chè 2 cher(e), chéri(e), mon (ma) cher(e)
Raphaël Confiant (2007). Dictionnaire du créole martiniquais. Ibis Rouge.
Confiant gives ‘sé’ as the Martiniquian for ‘c’est’, which casts doubt on the theory in the post that ‘ché’ might mean ‘c’est’.
The two answers that have been posted so far are exclusively based on Jane Eyre. An Autobiography and assume that that story's narrator is reliable. Of course, one may well ask to what extent her account of her husband's past can be relied upon, since she does not condemn the cruelty of his imprisonment of his first wife.
In Chapter XXVI, Richard Mason's ...