Here's how the answer breaks down in my experience:
If it's for a commercial, and you've had the copy (the lines) for a day or more before the audition, then you are expected to be letter perfect. If the copy is not available until you show up for the audition, and it's only a line or two, you will be expected to be letter perfect. If it's a lot of lines, you'll be expected to be close to perfect. I know that sounds crazy, but that's what they will expect in most cases. There is usually very time in the commercial world to learn lines, so the person who is quick with memorization has an advantage. Yes, you obviously have to be a good commercial actor, but remember, they often call in a large number of actors to audition, so there's going to be ones that are both good actors, and fast memorizers.
The general rule of thumb is - the faster the production shoots, the faster they want those lines memorized. Makes sense, yeah?
TV is kind of like commercials - it shoots fast, but the skills of the actor become a little more important than just memorization. As long as you don't screw up the lines too badly, your acting can make up for some mistakes. At an audition for a recurring role on a popular show, I screwed up the few lines I had both times I read and still got the part! Why? - well, maybe because I looked like the actor whose brother I was playing. Or maybe because I'm brilliant. But probably the look-alike thing.
With films, you get more time. You will usually get the sides a few days ahead of your audition, and as long as you give a strong reading as the character, the words don't have to be perfect. You'll usually have plenty of time to work on the lines before the shooting starts, and the director and producer know that.
And a play? Most plays are in rehearsal for at least 3-4 weeks (sometimes longer), so like a film, it's more about your interpretation of the character than it is about memorization.
If you email me, I'll send you - free - some tips on memorizing lines. Just put "memorizing" in the subject line. Or buy my audition book - it's got a chapter on memorizing.