I think it’s helpful to do any kind of speaking that mimics a real conversation. Some ways to make it easier are practicing a written or planned conversation, and maybe changing one or two parts with some words that are unique to the student. Or asking questions you agree on in advance. When I worked with children we did a lot of “Do you like x?” “Yes I do/No I don’t. I like Y.”

It doesn’t feel like a conversation to an educated adult but we’ve all had that conversation with a picky friend:

”Where do you want to eat?”

”Well, I don’t like tomatoes.”

”OK, do you like hamburgers?”

”No, I don’t like bread.”

”OK, shall we eat Chinese food?”

”No, I don’t like Chinese food.”

...etc... This can be a conversation too!

From time to time, there are indeed students who want to learn through conversation but cannot understand or respond when one utters even the most basic greeting, for example, "Hello, how are you?" in their target language. This seems unrealistic to me, but it does happen, thanks to the popular and incorrect idea that all you have to do to learn a language is speak it. A good teacher can maintain a conversation at a low level, but there are limits. If someone doesn't understand basic greetings, even the best teacher cannot work magic.

In my opinion, until a student can understand practically everything said to them about everyday matters, their interests, and commonly discussed topics in the culture (so, more or less an A2/B1 level), speaking practice <em>only</em> is neither productive nor enjoyable, and will most likely result in frustration, not the desired progress.

I’d agree with the answers saying A2/B1 level, but I also want to say that I do not think of conversation practice as the only form of learning a language student should be doing. It’s ideally a supplement, or at least a guide to vocabulary expansion and grammar review.

In my opinion we have to start with basic dialogues when we haven't a deep knowledge, but it isn't good to wait too long to talk about anything. I think that waiting too long will be too boring and a simple conversation should be proposed quite soon. Anyway it depends on goals which we are looking for. For exemple I have to improve my basic level, but if I cannot try to talk I'll never better and mainly I risk to lose confidence. If someone knows only very few words, perhaps he hopes to be guided step by step through a conversation because he doesn't want to speak only about booking hotels, shopping in different stores or something like that. I think that students desire to achieve concrete instruments to talk about their lives quite quickly, we could consider conversation as dialogues at different levels.

I'm not a teacher, so my experience comes from free language exchanges.

I've had exchanges with beginners (A1 level) whose only goal was to practice a few Italian sentences to interact with waiters and hotel receptionists.

Things become more interesting at A2-B1 level: you can discuss about a wider range of subjects, only needing some hints on a few words, pronunciations or grammar rules.

B2-C1 is almost like talking to a native: sometimes they stumble on some of the trickier parts, like irregular verbs or subjunctive mood.

Some of your students could even learn the language on their own, and use your weekly lesson to check their progress and pinpoint the parts needing more work.