How long do you plan on staying with this company?

What they are really asking

Hiring and training someone is expensive, so the interviewer wants to know whether you'll still be around after the first year. It's a risk question. They're not expecting a promise, and they'd be suspicious if you gave one. What they want is a sense that you're thinking about a future here, not using the job as a stepping stone to somewhere else.

The traps are at both ends. "Forever" sounds unrealistic. "About a year" sounds like you're already planning to leave. The honest middle is a commitment with a condition: you'll stay as long as you're growing and useful, and here's what that could look like over a few years.

A shape that works

Commitment, condition, picture. Say that you're looking for somewhere to stay for a good while. Add the honest condition, which is usually growth or contribution. Then paint a short picture of what the next few years here might look like, based on what you know about the role. That picture is what makes the commitment believable.