フライトの何分前までにチェックインしなければいけないですか？って英語でなんて言うの？

"How many minutes do I have before I have to~"というパターンになります。

This means that you want to have an idea when you should arrive to the airport before check in closes. Depending with the country you can also check in online and you will have to pass that stage when you get to the airport.

Airlines have check in times that can be as long as two hours in some cases. Usually, if a passenger delays to to check in, he/she misses the flight and has to be put on another flight. This can cause a traveler, especially a business traveler, to miss some important appointments at his/her destination unless he/she is able to reschedule them. So, it is extremely important to inquire about the check in times and be very familiar with them.
So, you may ask:
How long do I have to check in before the flight time?
or
My flight will take off at 2.00 pm, by what time should I check in?

You can ask the first question if you want to know the earliest available time to check in for your flight. The second question is for if you want to know the latest time you are allowed to check in. The third question allows the staff to respond with the time they think is best for you to check in.

'when can i check in from?' asks for a time when you can start checking in.

'How long before my flight can I check in?' This asks how many minutes before a flight you can check in.

'When does check in open?' asks what time you can check in from.

Any of these queries will suffice.

We must "check in to let the airline know we are ready to fly on time and have reached the airport OK.
If we miss our "check in time" we may NOT be allowed to board the plane..
So it is urgent that we establish the time-line and be sure of our correct
"check-in time"

If you want to have lots of time to be able to have a relaxing journey to the airport, you should leave your house early and arrive at the airport several hours before your flight departs. You can use this first phrase to ask the agent at the check-in desk how much time you have available until the check-in desk closes. Sometimes, you don't need to go to the check-in desk because you may have already done the check-in online. You would go to the desk only if you want to check in a bag,

We can use the second sentence if we need to know how much time we generally we need to allocate to ourselves so we don't arrive to late to check in.

1. When does check in close for my flight?
   It's important to know when check-in closes so that you
   can know how much time you have to work with.
   "Work with" in this instance means how much time you
   have left to browse the shops, have a bite to eat or take a nap
   before you have to check-in.

   2.When can I check in for flight SA354?
   If you ask the airport staff this question then they
   can specify exactly when check-in opens and when it closes.
   You can then decide when you will return to the desk to
   check-in, or alternately you can opt (choose) to do it online.

When can I check in before my flight?
How long before my flight can I check in?
When does check in open up?

All of these should help your out when you want to find out when check in opens up for a flight. Also, I would use the third one if I were to ask about check in.

I hope this helps.
Have a great day.
Will

If you want to ask someone how long before your flight you must check in, you can say something like "How many hours before my flight must I check in?" or "How long before my departure time must I check in?". These are both easy and polite ways to express this to someone.