What is the difference between
"Sleep for at least 7 hours" and "Sleep at least 7 hours" ?

Question

What is the difference between 
"Sleep for at least 7 hours" and  "Sleep at least 7 hours" ?

Answer

They are the same and equally correct.

Answer

They are the same. You can use both.

Answer

'Sleep for at least 7 hours' is the grammatically correct way to say this phrase.

Sometimes the other way is used in speech but it's just laziness. They mean the same thing.