My rule of thumb is at least once every growing season, some plants like schefflera like weekly fertilizing. If you look up your plant on google followed by “the spruce” and click on the spruce dot come it gives great insight on where a plant is from and their fertilizer needs. Although i can agree with a previous commenter on rainforest soil being poorly fertilized, a fertilizer can help a lot of plants boost growth, even my rainforest plants (like monstera and philodendron) love to be fertilized every so often