How long is too long? (pumping)

bornmommy

...to be hooked up to a pump?  Is there a limit as to how long, as in minutes per session, you should be pumping for?  I like to get at least 4 ozs from each breast per pumping session.  Its not always possible, but when it is possible I'll stay hooked up until I do.  (Putting around 40/45 minutes on an active pump.)  Just curious if this is "okay" or not?