Hi Manos, I haven't direct
experience, but it sounds like the traps are not quantitative. According to
Univeristy of California Integrated Pest Management program
While there is no relationship between fruit damage and the number of insects found in traps, surveying trap catches can evaluate treatment efficacy by comparing trap catches before and after treatment (when less flies should be caught).
So you probably will get a feeling for how many insects cause how much damage in your location in time. Not very helpful I'm afraid. Maybe others using the traps might have more experience to share.