"Until I come back" is the correct usage. Using "until I'll be back" isn't grammatically correct. When talking about future events with words like "until," "when," "after," etc., we use the present simple tense, not the future tense.

"Until I'll be back" and "until I come back" are both grammatically correct but have altogether different meanings. Suppose, for example, that you plan to come back tomorrow at noon. In that case, "until I come back" means "until tomorrow at noon". However, because it is true right now at this instant that "I'll be back", "until I'll be back" means "until right now at this instant". It is a construction that is not often useful.

In this context, you need to use the present tense. The correct sentences would be: "I'm just going out. I won't be very long. Wait here until I come back." You don't need to use the past tense in this case. The present tense is used after "until" to refer to future events.

I’m going out now. I won’t be very long. Wait here until I get back. Most natural: I’m leaving now. I’ll be back soon. Wait here until then.