Translation:I believe he thought for five minutes.
MandolinFace: first of all with 'credo' you need a subjunctive form, so 'abbia'. Next, 'pensava' is imperfect tense to describe an habitual action in the past, whereas to say 'he thought...' describes a 1 time action in the past that's over and done with. In other words it's not as though 'he used to think for 5 minutes, every once in a whille." I think those are the reasons why 'pensava' isn't appropriate.