How many times should we check mirrors while driving?

[Reza]

Which mirrors and how often should someone check their mirrors while driving in a straight line?
While I think the attention should be on the front, but it seems it is required to check mirrors constantly,
especially while driving on the highway.
Do we need to only check the back mirror? If there is no lane to my right it does not make sense to check the
right mirror. So, is it only the left mirror and the back mirror while I am driving on the right most lane?

[Inazuma]

Which mirrors and how often should someone check their mirrors while driving in a straight line?

Ideally, you should know what is going on behind and beside you, at all times

If you get passed and that's the first time you've seen that vehicle, or it surprises you, you're checking your mirrors far too little.
Ideally, you will know beforehand when a vehicle will be about to pass you

While I think the attention should be on the front

That's where your attention should be focused mostly, but not exclusively

, but it seems it is required to check mirrors constantly,
especially while driving on the highway.

Exaggerate a bit on the exam, and show you're looking in the mirrors

Do we need to only check the back mirror?

No
Use them all
Though the internal and left mirrors will be most useful

If there is no lane to my right it does not make sense to check the right mirror.

Emergency vehicles or road pirates might approach on the shoulder