Water

All plants need water, some more than others. Generally speaking, most orchids don't need a lot of water compared to most other plants, as many are adapted to sporadic and unpredictable rainfall by storing water in their leaves, stems, pseudobulbs, and roots. Controlling moisture levels is especially important for orchids that live above ground level as their roots are adapted to the natural watering cycles of wet vs dry, and need the dry periods and exposure to air in order to circulate away from the roots.

Excessive watering can lead to fungal and bacterial rot, and accelerate the breakdown of media. In many species it can lead to markings on the leaves or the release of dew from the leaves which can attract fungal diseases, and insects such as ants that can bring other pests such as scale and aphids with them. In severe cases it can kill an orchid by destroying all its roots, or by inviting rot to enter the plant when the base is exposed to wet medium for an extended period of time.

It is comparatively more difficult to kill an orchid by underwatering it, at least in mild conditions - sure they may shrivel up and look a bit unsightly for a few weeks, but they almost certainly bounce back. Underwatering also has some benefits:

• It encourages root growth
• It deters pests that thrive in moist conditions
• For some varieties, it can trigger bud production during cold seasons

There are times when watering should be more tightly maintained:

• When humidity is low and temperature or airflow are high
• When buds are being produced or they are flowering
• For orchids that are mounted, especially in warmer seasons

It cannot be prescribed to water orchids a certain amount every x days as it really depends on the temperature, amount of airflow, relative humidity, type of media being used, and age or health of the plant. You'd get the feel of it after a while and watch for the signs: 

As a general rule, it is easy to give an orchid water, but much more difficult to take that water away. And as you can see above there are many more noticable implications of overwatering than underwatering, so look out for the signs and correct as you go.

Recovering Overwatered Orchids

Because the roots are often the first thing to go when overwatering, it's important to first take care of them. Pull the orchid out of the media and leave it bare for a few days until it has dried. Cut the dead roots - dead roots will not have any body to them, they will feel hollow compared to alive roots which will feel somewhat solid and firm.

Then above the 'waist' - for sympodial orchids with bulbs, check for rotten bulbs and cut them. In the case of monopodial orchids it can be a case of wait and see, especially with crown rot - cut out what you can from the leaves, keep them dry as possible and monitor. Often the crown may die and then new growth can appear a few weeks later. The key is to keep the roots clean and provide lots of air around them, hang the orchid on some string or suspend it within a pot using chopsticks or skewers, monitor until the rot has subsided and new root growth appears.

Once things are looking better, spray a solution of seaweed extract (diluted following instructions from manufacturer), or a tea solution (a cold cup of black tea will do). The tannins in either encourage root growth. Adding a cinnamon stick to the bottle does no harm and is claimed to be antifungal. Another approach is to let them sit in a tea or seaweed solution by day and let them dry at night.

Recovering Underwatered Orchids

If an orchid is underwatered, it is usually in need of some simple TLC.

First off, don't water hot stressed orchids - if hot, transfer the orchid to a cool shady area, give it a while to acclimate, and then water. This will help prevent any bacterial issues.

If the orchid looks water stressed but is receiving enough water, it can be an issue with the roots. Give it a nudge to see if it is standing strong in the media. If not, then the media may have broken down, or the roots may have rotted, been damaged or eaten. In this case it is best to repot as soon as possible and then transfer to cool shady area, leave it for a day or two, then water.