"tell me about the last time that happened" is better than "how often does that happen?" How often doesn't tell you much. Plus it's unlikely to be accurate - p

"tell me about the last time that happened" is better than "how often does that happen?" How often doesn't tell you much. Plus it's unlikely to be accurate - people just don't keep track of things closely enough before you ask a question like this. But "tell me about the last time" is specific and someone can tell you a story. Then you can dig into it.