Which sentence is the best?

How long will it take FOR dinner be ready?  This is the one to go with.
How long will it take TO dinner be ready?   This is wrong, "to" has no place there.
When will dinner be ready?                  Also perfectly fine to use.
When is dinner going to be ready?           And also just fine to use.

The second may be from:"How long will it take TO make dinner.

(1), (3) and (4) all sound good to me, though they have slightly different meanings. (2) is incorrect.

(1) is asking for the amount of time needed to finish making dinner. The answer could be "a half an hour." Another very conversational of asking the same question as (1) is, "How long until dinner is ready?"

edit - your phrasing of (1) is slightly off. (1) should say: How long will it take for dinner TO be ready?

(3) and (4) mean the same thing - what time (on the clock) will dinner be ready? The answer here could be "7 o'clock."