There are a variety of synonyms that can be used to describe a "long period" of time. Some examples include: extended duration, prolonged interval, drawn-out span, protracted timeline, lengthy stretch, and extended spell. Other options might include extended period, prolonged phase, prolonged term, and prolonged duration. Each of these phrases can help to convey a sense of time that goes beyond the typical or expected, indicating an amount of time that is significant, substantial, or even overwhelming. Whether you are writing about a long period of history, a protracted illness, or a lengthy project, these synonyms can help you to add depth, nuance, and variety to your writing.