In the comments, @FumbleFingers highlighted some changes needed to your examples.
- ❌ No: 1. How long {it's} going to take until {it'll} be ready?
- ✔️ Yes: 1. How long is it going to take until it is ready?
Note that it's -- "it is" -- needs to be inverted to "is it", because it's a question (see more at this article).
- ❌ No: 2. How much time remain / left until it'll be ready?
- ✔️ Yes: 2. How much time is left until it is ready?
- ✔️ Yes: 2. How much time remains until it is ready?
More suggestions
You could replace "it" with the food, lunch, or dinner.
To this they may say something like "in 15 minutes". This seems very informal to me.
- ✔️ Yes: 5. When will the food be ready?
Again, seems very very informal.
Eat is fine. Notice that the verb "dine" isn't used this way in conversational English.