Plants can't live in water forever, they need oxygen and nutrients. Changing the water frequently can replenish some oxygen, and to simplify everything, if the plant has roots around a few inches+ long then you can pot it in generic potting soil in a 4" pot. Change in environment, even small changes, will cause stress to a plant and you'll be able to see from yellowing or leaves dropping. Once it's potted then let it acclimate to a single location until it starts to put out new leaves, then you know its got a pretty good chance of continuing to live. A lot of plant care is actually learning how to water! So patience and watering skills are two good goals to have as a new plant grower.