The Holga's shutter speed
The Holga has a shutter speed of approximately 1/100th of a second, although each Holga is different so yours may be slightly faster or slower. Due to the way the shutter is constructed, it's probable that the shutter speed may change over time. This is due to the fact that the shutter mechanism consists of a simple spring-type piece of metal, which over time may loosen and cause slower shutter speeds.
If you notice your pictures becoming lighter in normal sunny conditions, it may be that your shutter is becoming slower. You should adjust your film choice accordingly.
Long exposures
New type 'N' Holgas feature a bulb mode which allow you to keep the shutter open indefinitely. For long exposures you need to be aware of something called Reciprocity Failure - see the long exposures section for more details.
Testing shutter speed
If you regularly use slide film, you may find yourself needing to be more precise about your exposure times. If you are handy with a soldering iron, you can quickly and cheaply build a digital shutter speed tester. You can find a copy of the schematic and build instructions on photo.net.
Once you've built tester, you plug the tester into your line-in port on your computer, and tape it to the back of your Holga. Next load up a sound editor such as Audacity. Set the editor to record and record and click your shutter a few times. You can now zoom in and measure your shutter speed to the nearest millisecond.