@jenetix, you can get a free manual here:
Once a year is fine if you take it out for a brisk drive of at least 20 miles once a month. @dagosa is right, more frequent oil and filter changes is a waste of money. That 20 mile drive can be a coffee date at a country inn.
I rented a car (Vauxhall from GM Europe) ) in England and it specified 20,000 mile intervals or ONCE A YEAR oil changes.
Car companies have to cover their buns with respect to warranty claims and two oil changes per year is not going to bankrupt the average car owner.
Our Corolla now only goes 4000 miles per year and it gets 3 oil changes every 2 years. The oil is always clear as corn oil when the car goes in.