How often should I water my orchid if it's drying out?

It's not so much about the frequency in days that matters, but rather the state of the substrate and the ambient humidity. Orchids prefer to dry out quite a bit between waterings, so the ideal thing is to stick your finger in the pot and water only when you notice it's almost dry, which could be a week or ten days depending on where you live and how much light it gets. If the pot has drainage holes and you're using orchid-specific substrate, basically you wait for it to dry well and then water thoroughly; the most common mistake is watering too frequently, which is what kills them.

Orchids hate excess water, so it's better to water once a week or when the soil is completely dry to the touch. If it keeps drying out, check that it has good humidity and indirect light, because sometimes the problem isn't how often you water but the conditions where you're keeping it.