I did this for a cabinet full of bonsi trees. After doing some research, I found that 40 seconds of water each day was perfect for the plants. Pot sizes, evaporation rate, amount of water delivered to each plant per second through the tubes, and water requirements for the plants was the key to the time decision. Then, the timer was connected to a small pump and set - no more concerns about watering.
A sensor to water the plant when it is dry will keep the plant damp but most plants don't want to be constantly damp and their roots will rot. A little research about the specific plants you are watering might reveal that they want to be watered once-a-week with 1/4 cup of water (for instance) and you can design your timer for that frequency/amount. The sensor project described previously might help with the research part of the project.