I have never heard that there is any limit or guideline on fees the inmobiliarias can charge. May be, it wouldn't surprise me, but I've never heard of it.
But to me, the set pattern is between one and two months value of first month's rent as commission. I've never seen anything different on long term rentals over the 7-some-odd years I've been renting long term.
Seems the smaller companies are 1 to 1.5 months and companies like Remax (who I rented my latest apartment from) charge 2 months. But I've also found, with one exception, that the bigger companies work harder to find you something than the smaller companies do. With one exception among the smaller companies, I called them for a specific apartment and went to see it, and I never got a call back from them on other apartments that became available while I was looking. With Remax and the one small company exception, I got a multitude of emails related to new possibilities of apartments that met my guidelines.
I never was offered anything as a percentage of the total lease or anything like that with any inmobiliaria, always between 1 and 2 months. May have been that others with whom I never tried to make a deal would have offered something different.