In some situations could result in a 30 minute speech though.

That has got to be a very rare occurrence. Asking first is in my opinion the best advice. If in rare situations the person is giving a long winded or incoherent answer it should be okay to interrupt, but to avoid asking them what they tried on the off chance they are a poor communicator seems to me a bad tactic.

That's unfortunate but if the user has done something that takes 30 minutes to explain it's probably worth listening.