Wait time depends on the strain, but from seed its going to take you at LEAST 60 days before you reach peak harvest, and then figure at LEAST another two weeks before you have something cured properly. Realistically you can probably expect to take about 3-4 months from seed to smoke. You can cut corners and harvest early, and not cure properly, but you're just going to be hurting yourself with herb potentially quite a bit less potent and smokable than it could be.
How much weed can you get off one plant? Multiple POUNDS *IF* you grow a big high-yielding strain outdoors in the right climate.
How much weed will YOU get off YOUR two plants in a PC box? Frankly, for a first time grower, in a space that small, with only 1 small bulb, you'd be lucky to get 1/2 an ounce.
With the right strain and the right setup and the right care, you might be able to pull a full ounce out of something the size of a PC box, but I certainly wouldn't expect that.
Incidentally, you say you have a 100watt light bulb. Regular incandescent light bulbs are a VERY poor choice for growing plants. Not only do they put a spectrum of light that plants can't use well, but they also generate a huge amount of heat relative to the amount of light they put out. I would not expect very good results from an incandescent bulb; any plants grown under one will be sickly, tend to stretch upwards, and give poor yield.
If you want to go in a teensy setup cheap, try some 26 watt compact fluorescent lights. Two of those will put out more than double the effective light of a 100W incandescent with half the heat production. If you have a bit more scratch and knowhow, you might be able to squeeze a 70 watt HPS light into a PC box with better quality output.
