How often should I water my terrarium?
That all depends on your plants. There is no one "recipe" since the needs of a bog terr will be vastly different than that of one catering to plants from arid regions.
Generally speaking -- for terrs w/ the more common/typical houseplants -- some condensation on the glass is okay, but if you are getting lots of water droplets condensing and running down the sides of the terr then you have things too wet. Open the terrarium to let some of it evaporate. Watering/misting may be done when the glass remains dry near the soil line (no condensation at all). If the surface soil is dry but still moist a little deeper down, you should still be fine water-wise. The goal w/ the major of plants people use is to keep the soil evenly moist -- not soggy.
It should also be noted that one should have a very light hand when watering -- it is always easier to add more water later if needed than it is to remove excess water! If it comes to erring on the side of caution, most plants seem to be able to handle soil a little too dry than too wet.