How much you lack?

It means, "how much do you have remaining?"

If you're waiting on someone to do something, you may ask them this to get an idea of how much longer it will take for them to complete their work.

In the Southern United States, the word "like" will be used in place of "lack"; see: "How much you like?"

John: I can't come over until I've finished painting my fence.
Mary: How much you lack?
John: I'm nearly done. I have about 30 minutes left.

lacking in the penile region

The act of not being up to par with one's standards of the penis

Man, Baxter is really lacking in the penile region.