。
I study for hours at night.
Literal
Night [at-に] many-hours-[emphasis-も] study-[polite-ます].
~も attached to a quantity phrase 何時間 ('many hours') adds emphasis — 'as much as X, (a whopping) X.' The pattern quantity+も is the standard way to intensify quantities: 10 人も ('as many as 10 people'), 3日も ('for 3 whole days').