When the soil gets dry about 1/2 inch deep it is time to water. Be careful not to over water though! Better to go a little too dry than to over water.

That's actually harder to answer than you'd think. Is it sandy soil where water flows right through quickly, leaving it dry? It is clay soil where it just sits there until it evaporates? Is it a loamy soil where it takes a little while to drain through so the plant has time to absorb some, then the soil remains damp? It is windy or fairly sheltered? Frequent winds evaporate more water from the plant and soil, faster, than fewer winds. Is there a lot of hardscape around them, increasing reflected, radiated heat, making them need more water? Or, is the bed they're in surrounded with lawns which reduce the water needs? There are actually several more variables which can affect how much water, how frequently, the plants may need. Can you give more particulars about where they are planted; any walls or sidewalks they may be up against; where you live so we have an idea of your heat/wind/aridity, etc.; soil type; exposure, etc.? Without more information, any suggestions you'll get are shots in the dark and may not be appropriate. Kim

As much as they need. When the plants are small, water when the first inch or so of the top soil is dry, this could be everyday if the weather is warm or windy. As they grow, you water at greater intervals and you water deeper to encourage the roots to grow down and out - tomato roots can extend 6' except when in containers of course. If you water too much, the plants produce vegetative growth, stems and foliage. If there is any risk that their lives are going to end (stress) i.e. drought, flowers and fruit are produced. So don't make them too happy, but don't stress them so far that they can't recover. Growing them fast (lots of water and fertilizer) can lead to predatory insects, wind breakage and collapse, growing them hard (moderate water and fertilizer) leads to longer lived plants and reproduction (flowers/fruits). So, there is no substitute for daily observation and accessment of the weather and soil conditions. Also, veggies and fruit given the time to grow slowly absorb more minerals from the soil and have a higher nutritional content. Isn't that a good reason to have a home garden (good taste too)?

just a thought: poinsettia is also Euphorbia and if allowed to dry completely also will drop leaves. just barely moist is what it likes. I read the same about Euphorbia tithymaloides. Although when in ground it behaves more like a real succulent and can tolerate dryness. It is a native FL plant and grows in dry tropical forests in central America. so very hot and high humidity too. I've seen it growing outside in central FL in great thickets in dappled shade. if you give it good hot sunny window you can probably water it more. that said...Poinsettias drop leaves when too wet too - so..a lot depends on how water retentive your soil mix is. I can easily keep Poinsettia going in self-watering pot, but with half perlite for aeration. I also grow other succulents like Jades and Sansevieria and hawortias on top wicks (wicks inserted around the perimeter of the pot to wick less, as opposed to bottom wicks). my jades drop very few leaves when grown like that. when I don't wick and allow them to get bone dry, that's when they drop most.. so you need to observe and experiment a bit and water very carefully only when at least half the pot is dry. what sort of mix is it in?