Virtually all plants used indoors as "houseplants" are tropical in origin. The ones that are commonly used indoors have proved to be popular because among other things they do not require high humidity even though they are native to humid regions.
FLF's tend to grow in irregular spurts when indoors but they do not have dormant or semi-dormant seasonal variations. From what you have written, I suspect that most of the leaf spotting is because you have been underwatering it. Whoever wrote to water it once per month has no idea what they are talking about and have badly misled you. Your tree will need a thorough watering as soon as the SURFACE of the soil feels dry. Given its location and pot size once per week would be about right.
Does the pot have a drain hole?
The light that yours receives in that location is less than ideal because the tree is getting larger than the window. Do you have a sunnier location for it?
