How much water is too little?

Every plant is different. They are each in different rooms, households, pots and soils. It is impossible to give firm rules.

Some plants are bog plants and want a pot sitting in water, some are desert plants and want only occasional watering. Most fall in between the two.

Symptoms of a watering problem – too much or too little include:
Leaves drop all at once
Leaves drop slowly beginning with oldest leaves
New leaves are smaller in size than older leaves
Leaf tips are brown
Leaves have brown spots
Plant wilts
Little to no new growth on the plant

Symptoms specific to under watering your plant:
Leafs curl down
Accordion type creases appear in leaves ( especially on orchids )
Vertical splits appear on leaves
Thick leaved plants have wrinkles in the leaves
Pseudo bulb is wrinkled ( on orchids )

Ok so the plant is dying of thirst. Not a big deal, most will bounce right back with proper watering.

Now find out just how much watering that particular plant would like. Check it by putting a finger in the soil. Do not water by a schedule.