Okay, so here's the thing.
When you go shopping around Christmastime, it's going to take awhile. Before you go to the mall, you should mentally prepare yourself.
I'm talking to you, guys.
When you go to the mall with your girlfriend/wife/whoever, plan on being there for at least an hour. No "five minute" trip is actually going to take five minutes because it actually means "five minutes under ideal circumstances." So a trip that could have taken five minutes actually ends up taking a lot longer because you can't find what you're looking for, you lost your coupon, you have to wade through crowds of people, etc.
So it's not that she's lying. She's just underestimating.
Either way, just keep this in mind when she invites you to go to the mall with her.
Now. If you have arrived at the mall and you're in a store, DO NOT complain nonstop about being in the mall! You're already here and complaining isn't going to make her shop any faster. All it's really going to accomplish is making her—and anyone who overhears the two of you—more and more annoyed.
And taking this a step further, DO NOT, when she asks you, "What size do you think your mom wears," answer, "Geez, I don't know, I don't even want to be here!" She's here shopping for YOUR mother because you won't or don't feel capable and you're not only not being helpful, but also rushing her!