Cacti like about four hours of direct sunlight a day. Most cacti like the low humidity and warm temps for most of the year, water only when the soil has dried out from the last watering, which you can check by pressing your finger into the soil; the soil is dry if no soil particles cling to your finger when you remove it. For some cacti, you might need to water weekly, while for others, watering every two to three weeks is all that's needed. In the winter, cut back on water for desert-type cacti and water once a month. For tropical cacti, water regularly year-round but also keep the soil moist instead of dry while the plant is in flower.