Most people want to know how often they should water their bonsai tree and the answer they are looking for will always be something like – every other day or twice a week.  What they want is an answer that will not require any thought or attention.  As sure as shooting, if they're told to water it every two days, that's what they'll do.  It won't matter if it needs it or not. Two months from now, when their easy care Fig or Mini Jade Bonsai is turning yellow and dropping leaves because it's so waterlogged that the roots have rotted, they'll want a refund because they were – after all- just following the instructions they were given.

You Cannot Water a Bonsai on A Schedule

Do you want to know how often to water your bonsai?  There is only one honest answer and that's "When it starts to dry out", and depends on too many factors to account for.  How big is the pot? What kind of tree? What time of year? Where is it placed? How big is it? What type of soil is it in?  
But still it's not a tough question.  At first, you'll need to stick your finger in the pot and feel the soil to see if it needs water.  I still do this every time with every tree. Water when the soil about 1/2" beneath the surface feels dry.  For the majority of trees, if you're not sure err slightly to the dry side. But some trees like Serissa, Sageretia theezens, Pomegranate and Fukien Tea really hate to dry out, so if you're not certain, err on the side of more moisture.
Never water a tree that is obviously already wet and never, ever allow a bonsai to sit in a drip tray filled with water.  You'll kill it faster by not watering at all, but overwatering will kill a tree just as surely.

Don't Give Up Too Soon 

Just about everyone at one point or another will allow a tree to go too long without water.  Almost for sure you're going to drop a lot of leaves. Maybe they'll go crispy first – a sure sign that you waited too long to water.  If it's flowering, the flowers will fall.  If it has fruit, they'll drop.  It’s a natural stress reaction, but it doesn't necessarily mean your tree is dead.  Many bonsai will recover so just keep caring for it and give the leaves a few weeks to grow back. It's worth a try.

Observe Your Tree

If you're not going to bother paying any attention to it, maybe you shouldn't have a bonsai at all. But if you do want to learn, the first place to start after doing some reading on your tree is to learn to observe it.  How does the soil look when wet?  What does it look like dry?  If you have moss on the soil or the tree, how does moisture affect the appearance?  What about the leaves on your bonsai.  Look at your tree carefully after you water and you'll see that the leaves seem a little firmer, stand a little more upright.  Once you've been tending your tree for long enough, you'll be able to tell by looking at it from across the room if it needs water.  
Inspect your bonsai on a regular basis.  That's how you'll really get to know the tree – does that sound crazy?  Maybe it will to many people, but by knowing a tree you can see what's growing, how the shape it evolving, where new branches are starting out, or dying and you'll see trouble before it gets to the point of threatening the health of your bonsai.  You'll spot the aphids and mealy bugs and notice the fine webs of spider mites and get rid of them before they ever become a problem.  
And best of all, you'll really learn to appreciate how marvelously beautiful  your bonsai is.

How to Water

Water from the top or from the bottom. It's up to you.  Regardless of whether you water from the top or the bottom, make sure it gets a thorough soaking so that the full root zone benefits. Rain water is good. Tap water is ok, but I let it sit for twenty four hours to let the chlorine disperse.