It depends on many factors such as
- Whether your article is classified as an 'evergreen' one, meaning it's a topic that doesn't age, so people will search for it all year round.
-Whether your hub is written on a current event (it may get a spike in traffic, but will then phase out)
-Whether your hub is seasonal (Christmas, mother's day etc)
-Whether your main keyword is considered a 'competitve' keyword
Like Helena said, age plays an important part of traffic. Ive really only started writing on Hub Pages the last few months. The older the hub, the more traffic they tend to generate
There is really no hard and fast rule on what the typical traffic would be. I do find though, over time, you see a pattern amongst your hubs that show you which type of hubs will likely generate you a higher traffic.