Over the course of a year, the diplomatic bonus is added to the Current Diplomatic Rating for the assigned alien race. At the same time the alien race reduces the relationship by their Xenophobia. So if you had a diplomatic team rated at 150 assigned to a race that had a Xenophobia of 25, then over the course of a year, with no other effects taking place, the relationship would improve by 125.
Note that other incidents can also change the rating, eg the spotting of an alien ship when the current treaty does not permit it, or damage recieved from an alien unit.
Of course you wont know what the aliens Xenophobia rating is or what their Current Diplomatic Rating wrt you actually is - all you can do is guess based on their actions - if they're firing at you its a good indication that they dont have a particularly high opinion of you