How many days to spend at Universal Studios and Island Adventure?

2 days at least for both parks, 3 days to enjoy them more and go back to favorite rides. We got the fast pass last time and it was good for some rides but many of them were not crowded so we just walked on in in the mornings no pass needed.

I think 3 days is ideal. Things aren't as busy in October, so the Express Pass isn't as essential. But it's nice to have (and is cheaper when it's less busy, such as in October). If you stay at certain onsite hotels, the Express Pass is included with your package. if you want to ride the Hogwarts Express, then you'll need to have a park-to-park ticket.

each park is a good day, day and a half by itself. if you want to do Harry Potter stuff, that is divided up in to BOTH parks so getting the pass that allows you to go between the two as much as you want is best. the Front of the Line pass is WELL worth the money if you are not staying at an eligible resort hotel on site.

we plan 2 days in each with a down day built in so we can do the shopping thing in their version of Downtown Disney/Disney Springs.

October should be fairly slow so I wouldn't bother with the fast pass. I love Universal and IOA so I'd spend as much time as possible.

2 days would be ok, especially if you have Express Pass, 3 is ideal and lets you take things at a little slower pace.

3, and definitely get the fast pass