I have lots of PPC campaigns to monitor and I am wondering what do other internet professionals recommend changes to the campaigns? I would say to monitor at least for 3 months but always keep a vigilant eye from month to month.

Depends entirely on how much data a given campaign is bringing in.

For example, if a campaign only gets a few clicks a day, it will take many months to get reliable data on how to tweak it.

If, on the other hand, you are getting thousands of clicks a day, within weeks you will have a lot of data to work off.

It's not really a question of when, but how much.