(For) the entire day.
'I haven't watched TV (for) the entire day.'
'I haven't practiced English (for) the entire day.'

Is 'for' necessary in such contexts?

Question

(For) the entire day.
'I haven't watched TV (for) the entire day.'
'I haven't practiced English (for) the entire day.'

Is 'for' necessary in such contexts?

Answer

In front of 'the' it's optional. In front of 'an' it is necessary.

Answer

In spoken English "for" is not necessary, but you might consider adding it for more formal writing.