2.5nm either side at the IF, and reduces linearly down until intercept of the ILS OAS contours. The width is variable depending on the parameters of the ILS but is typically about 1.5NM at the FAF and reduces further down to the MAPt.
I cant seem to post an image. But PANS-OPS Figure II-1-1-2 gives you a good idea of how it works
Alpha