The time that it takes to cook a fillet is not long and it’s probably this one thing that terrifies most novice cooks. Under or overcook your fillet and you’ve wasted a lot of money. A rule of thumb is to cook it for 15-20 minutes per 500g.That will result in a medium fillet.Don’t forget to ‘stand’ the meat for 10-15 minutes when it comes out of the oven. rule of thumb is to cook it for 15-20 minutes per 500g. That will result in a medium fillet. Don’t forget to ‘stand’ the meat for 10-15 minutes when it comes out of the oven.