1. a period of time during which something occurs or lasts
a spell of good weather
This expression is usually used when discussing the weather, an illness, or a period of time spent living somewhere. For that reason it is most commonly heard in a social context. However, it can also refer to time spent working somewhere or at something. A 'spell of something,' is generally taken to mean a brief period of time, so you would need to prefix it with the word 'long' if making reference to a reasonably or relatively lengthy period of time. A similar expression is 'a bout of something.'