Your advices fon this speel

The Wizard's curse
Perdo mentem
Reduce the intelligence (-1) of the target each time the speel is cast.
Base 4 +2 voice Level : 10

How much the intelligence can be reduce -5, -6 ?

I am not sure about base 4. This base is to affect someone's judgement or sense of humour. It is affecting a part of their mind, not their actual characteristics.

If you look at the Creo Mentem guidelines - to improve a characteristic is base 30. I would say that this is a decent guideline for destroying characteristics too.

So your spell would become:
The Wizard's curse
Perdo Mentem 40
R: Voice, D: Momentary, T:Individual
Reduce the intelligence (-1) of the target each time the spell is cast.
Base 30, +2 voice Level : 40

Then we run into some problems. If you look at some of the Perdo Mentem spells, they have durations. After that, the targets mind reasserts itself. I can see many SGs ruling that this needs a ritual to have a permanent effect.

Note that anything at 30 or above is into the "one-shot kill" spell effects. In comparison, reducing someone's intelligence is less bad (arguably) than just shanking them. Therefore, it's probably not necessary to make the effect a ritual, in the same way that permanently killing someone doesn't have to be a ritual.

Also, check out the Perdo Corpus effects - it has a lvl 20 guideline that permanently damages senses or limbs that don't heal naturally (AM5th, pg. 133); you can also see that there are similar, yet easier effects, that DO heal like wounds. For example, the lvl 5 guideline allows you to "hamper a person without injuring them; for example, blur their eyesight. Heals as a Light Wound." So, I'd argue that reducing a characteristic is probably more akin to a lvl 20 effect. However, you could pull off a lvl 5 effect if you wanted it to heal. (Giving someone a concussion would probably temporarily reduce mental characteristics, for example.)

Also note that the "duration on Perdo effects" issue has morphed over time; originally in the core rulebook, only instant Perdo spells could be permanent. However, that has changed (either due to typos or intent), and now it seems to be able to go either way: Perdo effects with duration can either simply be suppression effects, of (essentially) instant Perdo effects cast continuously.

So, anyway - I would argue that the spell itself would need to be lvl 15 at least (Base 5, +2 Voice); that will grant you the ability to reduce a specified mental attribute by 1, and it heals like a light wound (although it's purely mental.) And yes, It could only get down to -5 using that spell. Any more, and you'd have to start using the lvl 30 "Destroy aspect of target" - as intelligence is an innate property of a human being.

To do it permanently, you'd need to use the base 20 version (+2 voice), for a lvl 30 effect.

ok i will think about it.