Explanation

Tina has been writing her report since two o'clock. She's written twelve pages. 

since + point: a "point" is a precise moment in time - 9 o'clock, 1st January, Monday.

while

for + period: a "period" is a duration of time - five minutes, two weeks, six years.

For means "from the beginning of the period to the end of the period"

ND13-07-2023