Litton LTN-101 FLAGSHIP (CRJ, Airbus) 7 minutes at "normal " temperatures. They were 10 minutes the same as the older LTN-90s. Note that the real arbiter is the temperature of the sensing platform itself. Once had to do a cold temp operational check - took 3/4 hour before the IRU came up.
Latitude has an effect as Cornish Jack rightly said but some IRUs won't even take an initial entry above 70 degrees N (once aligned they will work satisfactorily even flying circles around the pole).