I tried one of those one time, I didn't leave though I just used it. It kind of depends on how aerated the soil is to be honest. What they do is as the soil dries it will allow water to escape from the bulb. You don't want constant moisture in your soil though, it's really not the best way to grow in soil. Honestly, in my opinion, with soil you could pull off a three day stretch. Let them dry out now and give them a real good watering right before you leave. They should be mostly dry by the end of day two and ready for a new watering when you return.