Here are a few simple tricks to help ensure you can enjoy your Valentine's Day flowers for as long as possible. I personally follow these at home and had an arrangement for five weeks once.
Most houseplants need watering "about" once a week. Each plant will differ depending on its maturity, what kind of pot it is in, where the plant is placed in relation to sunlight/heat/circulating air, and what kind of plant it is. Plants placed in windows will obviously dry out faster than those exposed to little direct sunlight.
For best results I suggest you set a regular watering routine. Creating a routine will also help you remember when to water (or not). We check all our houseplants twice a week (Sunday and Wednesday) however, Sunday is our designated watering day. Some of our plants could easily go two weeks or more without water, but it is easiest for me to do them all on a consistent day rather than remember individual schedules (because that is not happening).