At least in old Renault V10 cars it was about 3 cm. But as Tom said, it could be customized per driver's preference.
More from Mauro Piccoli, chief of Brembo's racing department:
Alonso stomps on the pedal harder and generates more pressure on the system, we are talking of a pedal pressure in the 130-140kg range (+/-285-310lbs). It's similar to figures for Lewis or Jenson but Fernando is less aggressive. He likes to use a large diameter pump with a very short pedal. Pedal travel is in the order of 30-40mm, three and a half times that of the pump. Felipe prefers to manage lower pressures but as far as corner entry, there are no great differences.
Older info,somewhat related:
''Some drivers choose a different kind of pumping on brakes,'' said Vitaly Petrov, a driver at the Lotus Renault team. ''For example, we know that Fernando [Alonso] likes soft brakes, and some drivers like stiff brakes. I prefer to have very stiff brakes because I feel more in control of the locking in the rear and the front. This is why I can't drive with soft brakes, because maybe my left foot is too strong.''