Following up Mr.Ohm for DC circuits:
Power[watt] = Voltage[Volt] * Current[Amp],
so Current[Amp] = Power[watt] / Voltage[Volt],
so Current[Amp] = 29 / 12 = 2.42 [Amp]
and
Time[h] = Capacity[Ah] / Current[Amp]
Time[h] = 120 / 2.42 =~ 50[h]
EURECA! The answer is 50 hours...