It really depends on how much time you want to spend on both islands. If you just want a quick whirlwind tour you can do it in under 2 hours. But it is easy to spend nearly the whole day if you tour the museums, etc.
The total trip time to the islands is around 45 mins(total time in boat including all the stops ). Other than that it's up to you how much time you choose to spend there. The boats leave every 30 mins
Depends on time of year and whether flexi or reserve. If you go in holiday season on a flexi ticket you are unlikely to get to see both islands as there are not enough hours to accommodate for the queuing. Queuing can take anything from 1 hour (reserve) to 6 hours (flexi) to start your trip. Last boat to Ellis from Liberty is 3.30pm, or at least in Winter it is. If you are on a flexi ticket you will be rushed. If not, take your time Allocate until 5pm for the whole trip either way.
About 2 hours for one island, 4hours for both including boarding&security.
As Jeremy stated, expect no less than 2 hours. But it could easily (and you're better off) budgeting 4 hours or so.
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