How much a singer needs to practice truly depends on the person, the project, and the purpose.
The Person: Some singers want and need to practice every day, as it's such a strong part of their daily routine, their day would be incomplete without it. There are plenty of musicians who even take their instruments on vacation with them, as they want to keep their muscles in excellent shape. Other singers practice several times a week, and yet other singers practice only when they have a project or an upcoming audition, meaning they take days or weeks off without practicing. A singer's practice routine is very individual and unique. It takes creativity, time, and experimentation to develop a quality routine.
The Project: A project that's in a foreign language that's unfamiliar to the singer, say an English- and French-speaking singer learning a Russian opera or song, will take significantly longer to practice. Not only does a singer need to practice the melody and incorporate all the technical aspects of the piece, all while researching and learning about the history of it, but she needs to practice the Russian--first speaking, then speaking in rhythm, and then singing slowly and then up to tempo. Add memorization to the mix and you've got a lot of practicing!
The Purpose: The purpose of the practice session is also dependent on the singer and the project. If she's practicing for her own enjoyment, then anything goes! If she's practicing for a specific project, say the Russian opera mentioned above, then she will most likely be more concentrated, more focused, and more goal-oriented than when she's 'maintaining' her audition repertoire. When the purpose of practicing is maintaining audition repertoire, say 5 opera arias, then she will be experimenting more with keeping the pieces active and creative--remembering her memorization markers and feeling the text come alive again as she sings through the melody. We call this "re-creating" a piece and re-creation is in itself a main skill of performing.
In minutes, a practice session usually begins with a 5-15 minute warm-up, a chunk of practice time, and a cool-down of 2-3 minutes. Many singers practice for a total of 30-45 minutes at a time, and some singers can practice for even longer without tiring--their voices or their bodies. Many voice teachers advise practicing three times a day in 30-minute chunks. Personally, I'm a fan of a 45-55 minute practice session with a break in the middle.
And done right, a singer is going to be a bit tired after practicing (just as after a lesson or coaching)!