You will get different opinions on this. For my part, when doing a formal lesson, I correct immediately the error is made, then I give the correct version and get the student to repeat the correct version at least twice. HOwever, in a conversation class, unless the student makes a really bad error, I let almost everything go in order to allow conversation to flow. It's more important to me that the student becomes comfortable in simply talking out loud.
Note the amendments I made to your post. Please don't use non-standard words or phrases such as "Thnx".