In "The Rime of the Ancient Mariner," how long has the Mariner been under the curse?

The exact answer to this question is that we are not told.
I have carefully checked the poem again for any time references that could give us an
indication, but no specific time markers are given to us. The only thing that we can
deduce is that the Mariner has been under the curse for a long time. Note how he is
described in the penultimate stanza of the poem:

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The Mariner, whose eye is
bright,

Whose beard with age is
hoar,

Is
gone...

We can assume that
the Mariner would have been a young man when he had this experience, and his aged state
as described in this stanza suggests therefore that he has been wandering and sharing
his tale with the supernatural compulsion that he describes for a long time. Killing
albatrosses has its consequences, obviously. ...