I've done Island Lake to Elkhart Park in a day twice. Seems to end up around 7.5ish hours for me. The ups and downs from Island Lake to Big Seneca don't bother me that much, the scenery is nice and it's usually not too crowded at that point. Seems once you hit Big Seneca you hit the crowd. From Big Seneca to about Eklund still has plenty of ups and downs, not as bad, but you get down into the trees without much for views. It's not the most boring trail ever, but it's not my favorite.
The stretch of trail from Photographer's Point to Elkhart Park is what I have dubbed my most hated stretch of trail ever. Not because it's the worst trail to hike, but because it is SO BORING compared to the scenery you are coming from or headed to, and on the way back in it's about 6 miles of bland, boring trail between you and your car. It seems no matter what, my Winds trips always involve this trail and I've hiked that stretch many times and it just never fails to cause some kind of anguish in me. I even wrote a song about it:
This is the trail that never ends
it just goes on and on my friends
you know we started hiking it not knowing what it was
and we just kept on hiking it forever just because
this is the trail that never ends
(and so on)