One of the websites I use with students the most is Raz-kids.com. It's essentially the online version of
Other things that are great about the website is that you can monitor students' progress with respect to books read, levels passed, and minutes spent on the program. It's sometimes tricky to watch every computer screen all the time but you can easily use the teacher account to make sure students are on task and completing their assignments as assigned.
There are also reading benchmark tests at each level where students can read aloud and it is recorded for the teacher to assess. This reading assessment also includes a comprehension portion where students have to explain what happens in the story in their own words.
This program is well designed and also features a motivational point system to keep students engaged. As for time, I find that 30 minutes is usually about right. The actual lesson can take longer because it takes time to get students to logon to the computers and navigate to the website. Of course, this becomes much quicker after using it two or three times. It's also possible for students to access the website from anywhere so they can read at home for extra practice.