This phalaenopsis orchid prefers constant moisture at the roots.

First, most orchids we grow are epiphytic orchid that in the wild, grow on trees with roots exposed.  If you're a greenhouse grower, you can mount your orchids on bark to emulate conditions in nature, but most of us will be growing orchids in pots indoors. This is where watering becomes a problem. Most orchids receive frequent rainfall during their growing season--the roots are wet thoroughly, but because they are also exposed to the air dry quickly, before they are wet again. In pots, orchids roots are exposed to less air and do not dry as quickly. If you overwater them, the roots end up rotting and the plant will also go downhill. Orchid plants are only as good as their roots.

There are  three things to keep in mind in determining how often to water your plants.

1. The type of orchid. Some orchids like to be kept moist at all times, such as phalaenopsis orchids, while others like to dry out a bit between watering, such as cattleyas.

2. The light and temperature. In warmer brighter conditions(summer) orchids will need more frequent watering than in cooler low-light conditions(winter).

3. The type of pot and growing mix. Plastic pots retain more water than clay pots. Larger pots dry out more slowly than smaller ones. Some orchid potting mixes retain more water than others.

Basically, water your plants when they need it, not on some predetermined frequency such as once a week. Over time, buy observing your plants and their needs, you'll be able to set up a watering schedule for each season.