From the description of guidance orison:
The creature gets a +1 competence bonus on a single attack roll, saving throw, or skill check. It must choose to use the bonus before making the roll to which it applies.
Gathering information with the diplomacy skill takes d4 hours. Can guidance help with it, even though it has a duration of 1 minute (or until discharged)?
The key question for answering this seems to be: When gaining a bonus to a skill check, does the bonus have to persist for the length of time of the check, or only when the dice are rolled?
Supposing the bonus has to persist for the entire time, can one use several guidance orisons to cover a longer period of time? I am not aware of any rules support for this.