I thought there should be a time factor in this calculation
Both watt and horsepower measure power. They are the same thing, just with different units, hence the direct conversion.
You seem to be confusing power with energy. Power is an instantaneous value, while energy (e.g. joules or watt-hours) is what you use if you consume power for some time.
Note that the HP rating of a motor doesn't take into account efficiency, and so tells you only how much power the motor will produce, not how much it will consume.
how do I factor in the time to get the watts the motor consumes?
You don't. Time has nothing to do with how many watts the motor consumes. Watts is a measure of how much power the motor is consuming (or producing) at any given instant in time.
If you want to know how much energy the motor consumes, first you have to figure out how much power the motor is consuming, rather than how much it is producing (and a 0.5 HP motor doesn't produce 0.5 HP at all times...that's just its nominal maximum output), and then multiply that by the number of hours it consumes that much.
Having done that, you get a figure with the units as watt-hours, which is the amount of energy you've consumed. This will translate to e.g. a cost on your electric bill, or the capacity of a battery, etc.