Yeah, I think 18 months is reasonable.

Here is Apple.com a couple of months after Jobs returned: http://web.archive.org/web/19970404064352/http://www.apple.c...

There was an iterative change about 8 months into Jobs' return. http://web.archive.org/web/19971013114837/http://www4.apple....

Then things really started to get better about month 15: http://web.archive.org/web/19980509035420/http://www.apple.c...