The plants are going to depend on the species.Generally the plants last until they produce shoots/runners/bulbs to produce new plants,and rarely need to be replaced.
The water issue,is going to depend on the amount of wastes produced and the amount of filtration available.Heavily populated aquariums change faster than lightly populated aquariums.The amount of wastes produced,and the filters ability to reduce those wastes are going to determine the water change frequency,amount and intervals.
Not every aquarium is the same.Some change the water 2 times a week,some change it once a week,and any variation in between depending on waste and population levels.
How often do you test your water?
Generally I recommend to change the water depending on your
nitrate readings.The higher the nitrates,the more frequent the changes need to be.Less nitrates means you change the water less,whatever is needed to keep the parameters stable.
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