look for something (for)

[CarloSsS]

Do I need "for" in front of the time adverbial in the sentence below?

I was looking for my pen (for) about five minutes.

I think it sounds more natural with "for" but is it acceptable without it?

[bhaisahab]

Do I need "for" in front of the time adverbial in the sentence below?

I was looking for my pen (for) about five minutes.

I think it sounds more natural with "for" but is it acceptable without it?

No, it's not acceptable without "for".