Which one is OK?

Don't open it until I get back.
Don't open it by the time I get back.

Question

Which one is OK?

Don't open it until I get back.
Don't open it by the time I get back.

Answer

Yes the 1st phrase is the best!

Answer

The first one is fine.

The second one doesn't work. BY THE TIME somehow only works with a positive, or a request to do something (not a request NOT to do something).

Finish your homeword by the time I get back.
If you don't finish your homework by the time I get back, you can't watch TV.

BUT:

Don't eat dinner by the time I get back.    doesn't really make sense.

Answer

The first one is more natural and correct.

Answer

Mainly the first one.

The second one is subject to the situation.