I believe that the "Annals" say that he spent two Years of the Trees in captivity plus five years of the Sun. Since a Year of the Tree equals (approximately) ten years of the Sun, then he spent a total of twenty-five years in captivity. At least five of those years were spent on Thangorodrim since Years of the Sun began with the rising of the moon and that was also when Fingolfin arrive in M-e, and we know that Mae was already hanging at that point.

I've always figured that he spent his time in Angband until his brothers sent a definite decline to Morgoth's offer. This is just my interpretation, of course, not canon. :)

(And, Mom, if you're reading this, NC-17 means nobody under 17 allowed to read, right? Nobody said anything about writing. *ducks* :P)

Lol! *waves to your mum, if she happens upon this*