What affects how often I should water?
There are an incredible number of factors that affect how often you need to water each individual plant, including (but not limited to):
Plant Species
Different species have different watering needs -- one of the first things that you should learn about each and every plant, along with the lighting requirements, is what the watering needs of that plant species are! This could range from wanting to be kept consistently moist to wanting the soil to dry out completely between waterings.
Lighting
How much light you give your plant will determine how much water it needs, to some extent -- a plant in a highly-lit spot will drink more water, since it needs it to process the energy. This means that if you're having trouble keeping up with watering needs, you can move it to a different spot with different lighting to change them -- but it also means that if you move it to a different spot for any reason, the watering needs could change!
Soil Mix
The composition of your soil mixture will affect how well it retains moisture, as well as how quickly water evaporates from it -- the more air pockets in the soil mix, the more evaporation can take place!
Humidity
Humidity doesn't just affect the plant directly -- it also affects its watering! The majority of water leaving each pot isn't taken up by the plant -- it's actually evaporated. Humidity plays into this because humid air can't hold as much water as dry air -- so if your environment is particularly humid, you'll need to water less.
AIrflow
As mentioned above, most water loss is due to evaporation -- and one way to increase evaporation is to increase airflow (usually by adding a fan in the room somewhere)! What this does is make sure that when water evaporates from the soil, that water-laden air gets moved away and is replaced with (theoretically) drier air! Airflow is completely overlooked when it comes to houseplants, but I personally suggest always having a fan on in the room to help prevent overwatering.
Roots
If a plant is particularly root-bound, there isn't much soil left to hold water! This will mean that you'll need to water more frequently (and that you should really repot).
Temperature
A hot day will mean more evaporation, since warm air can hold more moisture than cool air!
Pot Material
Plastic or ceramic pots keep water in, but terracotta (or other porous, less common materials like concrete) will wick moisture away through the pot itself, leading to more frequent watering!