IS there a rule of thumb of how many minutes are necessary in order not to overcook pasta ?

It depends on the type of pasta. Look at the package instructions and taste every minute or so towards the end of the cooking time. When it is done, it should taste 'al dente'!

I would agree with the poster above. Pasta tends to get softer as it sits in the fridge or is reheated, so it is best to leave it a tad bit hard to avoid a soggy mess.