"They write for decades."
Translation:Eles escrevem por décadas.
Por can mean 'per' but it can also mean 'for' depending on the context. It seems to fit well when used in terms of time like in this example and other phrases like "for weeks", "for hours", etc.
Para is more simple because it is mostly only used when 'for' indicates a goal or destination. Para would be used in sentences like "I go to school" or "I came in order to solve my problem". So this example really doesn't fit the definition for para.
Por vs. para is tricky. Find an in-depth discussion here.