I think it uses roughly 1gb per hour average but depends on quality. Some recordings are closer to 2gb per hour.
The PS3 uses a standard 2.5" notebook hard drive which can be replaced easily, and without voiding the warranty (there's even instructions on how to do it in the manual).
So the best option is to buy the cheaper 120gb model then use the money saved to buy a 500gb+ hard drive and put that in yourself. That way you'll never have to worry about running out of space