Antelope Island is mostly driving. One road runs the length of it, the bison could be anywhere along that road, and that is really the shape of the day - you are hunting rather than visiting.
Budget half a day. There is not a great deal out there beyond the views, and the views are genuinely worth the drive.
$15 per vehicle for up to eight people, plus a $2 Davis County causeway fee. You pay the causeway fee even if you hold a Utah state parks pass.
Longer only if you are camping, hiking, or out there for something specific.
Getting there takes longer than you think
Once you are off the interstate there is nothing else down that road, and it runs for about forty-five minutes before you reach the guard gate. Then you still have the causeway to cross before you are on the island at all.
Worth knowing: you are not guaranteed to be driving over water. The lake has been low, and a good stretch of what you cross is exposed lake bed rather than anything you would call a lake.
The bison are the point, and you might not get one
We went for the bison, and we never got anywhere near one. My camera was zoomed to 34x and they were still just little black bits in the video. That is not a complaint about the island, it is what the island is - a large herd on a large piece of land, with no obligation to come near the road.
Which means the drive is the hunt. You are on the lookout the whole way along, and the pleasure is in the looking rather than in arriving anywhere.
There is not much out there, and that is the point
The views are good. Really good. But if you are not camping, or hiking, or there for something specific, half a day covers it, and I would not pretend otherwise.
There is a cluster of buildings once you are over - a visitor centre, a marina, a restaurant, and the usual hiking basics down to a hut. All of it is bare bones. We did not eat at the restaurant, so I cannot tell you whether it is any good or even reliably open.
We also barely got out of the car. We stopped at one viewpoint and walked a little higher for a clear view, and that was about the extent of it. The air felt thin and we were both struggling, although in July in Utah that may simply have been the heat.
The thing I did not expect to like was the salt. Where the water has pulled back it has built up along the edges, and up close that is more interesting than the view across the lake.
I am unlikely to go back. Not because it was bad, but because there is not quite enough of the sort of thing I like doing. That is a fit problem rather than a quality problem, and it is worth knowing before you drive out.
One road, a lot of looking, and no guarantees. If that sounds like your kind of day it is a good one.