[General] I studied (for) two hours yesterday.

[beachboy]

I studied for two hours last night.
I studied two hours last night.

Are both sentences correct?

 

[emsr2d2]

I would use only the version with "for".

 

[jutfrank]

I would say for is necessary.

 

[beachboy]

Do you think it's a mistake that some native speakers might make? How terrible would it sound?

 

[GoesStation]

Do you think it's a mistake that some native speakers might make? How terrible would it sound?

It doesn't sound terrible. An American might utter the sentence without for, or reduce the word to such an extent that it wouldn't be heard.