To ask your wife how long she will be home, you could say:

"Honey, do you have a rough estimate of when you'll be home?"

Or, you could say:

"Hey, sweetie, do you know approximately how long it will be before you're back?"

Or, you could be more direct and simply ask:

"When will you be home?"

It's important to communicate clearly and directly, but also to be respectful and considerate in your language.