'DEPUIS' => 'FOR' or 'SINCE' ?
He has been walking for four hours. He has been walking since six o'clock. for [] hours => we count since [] => starting point
He has been walking for four hours. He has been walking since six o'clock.
He has been walking for four hours.
He has been walking since six o'clock.
for [] hours => we count since [] => starting point
for [] hours => we count
since [] => starting point