很means very, 长 means long, 时间 means time. Add them together, you got 很长时间 means very long time.
I am not sure you need the 的 in the middle. Also, 常 means often(not the word you want to use here, even it sounds the same as 长).
Now, 了 means in the past, yes. But it doesn't means a long time has been passed. Instead, it means "it has been a long time" or "long time ago", depending on the context.
For example, If i ask how long you have been waiting in the line, you can reply by saying "很长时间了“.
That means you have been waiting for a long time.
Or, I can ask when was the last time you have visited China, and you can also answer with "很长时间了“.
That means it happened long time ago.
If you really want to say that a very long time has been passed, you would say "过了很长时间“
Just my 2 cents. Happy learning.