This answer suggested biking from Amsterdam to Marken island as a good tourist activity.
Is this a good idea? roughly how long would it take?
The seaside route, which is "flat as a pancake", is about 25 km (measured from central Amsterdam to the farthest point of the Marken island). This isn't the shortest route, but probably the one you want to take.
So, if you keep up a relatively quick pace (20-25 km/h) throughout, you could do it in a little more than an hour, but a more leisurely stroll, with a stop or two perhaps, would take 1.5 to 2 hours.
Is it a good idea? Yes, looks like it is. I haven't done it, but e.g. the page I linked to had this to say:
Very nice route, where you can ride near modern windmills and almost on the water. [...]
The town of Marken is really beautiful and recommended to see and photograph.
It is a day trip. As Jonik already points out, cycling without stopping would cost around 1.5 hours, but then you would miss most of the fun. The first stop you should consider is having a break in Durgerdam and take a drink a the riverside.
Then also consider going to Holysloot, which is a very nice short sidetrack.
And then offcourse this point where you experience the feeling of the land being below the waterline.
You can get a good taste of the trip through google streetview.