There really is no rule of thumb except for when-the-plant-needs-it. No fixed schedule will work as the needs change based on lots of variables - weather, temperature, size of container, age of plant, etc. etc. etc..

The worst gauge for watering is going by the surface of the soil - always leads to over-watering. And the best gauge for knowing when to water is your finger stuck deep in the soil down to the root level. If it is cool or damp no water is needed.

The ultimate goal is consistent soil moisture. Not "real moist" and not dry.

Dave

PS: edited to add - when in doubt don't water. Most people tend to over-water anyway but over-watering is more harmful for the plant than under-watering is.

This post was edited by digdirt on Wed, Apr 9, 14 at 20:24

I agree. CONSISTENT soil moisture.
Over watering can cause problems especially with soils that does not have good drainage. Furthermore , it can wash down the nutrients beyond the reach of plants.