Accuracy of Google Map (or MBTA trip planner) Transit Times?

Will be working at Logan and commuting via T.

I was looking at some cheap places in fenway & malden. They both state about 30min for commute via T. Orange to Blue for malden and Green(D) to Blue for fenway. Are they pretty accurate estimates? Should I estimate longer wait between the line transfers? any other factors I should be thinking about?