With most plants, you should water when the soil feels dry to the touch. You can gently stick your finger (up to the knuckle or so) in the soil to see how dry it is. For water lovers, water when the surface is dry; for succulents and drier plants, water when most of the soil feels dry.
No fancy gardening equipment is required to figure out whether or not you should water your houseplants, vegetable garden, or outdoor planters. Put your finger about two inches into the soil. If the soil feels moist, don't water it; if it feels dry or on the verge of being dry, then water it. Roots are the lifeline of the plant, deep in the soil and growing. But if they get waterlogged and rot, they can't absorb water, nutrients, or oxygen-all things a plant needs to grow.
Also, you can lift a potted plant (or carefully tilt or nudge the pot if it’s a big one) to gauge how wet the soil is. If you get a sense for its weight right after you water, you will have a base weight to compare it to as it dries out.
If the soil is dry and the leaves are wilting, the plant is likely thirsty. But wilting (and dropping and/or yellowing) leaves can also mean too much water.
If you're stingy with the H2O, that's also bad news. Thirsty plants get stressed and are more prone to pests and disease. Their growth will also slow down.
Most plants (but not all, because plants are wily things) will want more water in spring and summer, and less during their dormant period in fall and winter – you can tell their growth and dormant phases by when they are growing the most.
Because the variables that affect a plant’s thirst are ever changing, it’s best not to stick to a fixed schedule.
Since soggy leaves can invite disease and fungus, the best time to water is in the morning, giving the plant the daytime to dry out. For plants by windows that are accustomed to a lot of light, be careful of overwatering on cloudy days since their foliage will not dry out at the usual rate.
You can easily shortchange your plants by watering at the wrong time of day. The most optimal time is early morning evening when temperatures are at their coolest (early evening is okay, too). If you water plants at a time of day when the sun was hot and blazing, the water on leaves will evaporate too quickly, leaving plants parched until the next watering.