Is there a minimum charge even if I only stay a moment?

Yes. Every session charges at least 1 minute, even if it lasts less.

If you open a session, write, and leave after 20 seconds, you are charged a full minute. Time is rounded up to whole minutes; loose seconds are not billed separately.

That is why it pays to open a session once your question is clear, rather than to check whether the expert is connected. You can see their availability status on their profile without spending anything.

When you close, a few grace seconds are taken off the total calculation.