Why is my car using alot of engine oil?

I have a vauxhall astra coupe 2.2 bertone x reg (2000) and it seems to be using alot more oil than it should be. I havent noticed any smoke from it when i start my car nor have i noticed any oil leakage. I cant understand why it is using this much oil. I have put roughly about 2.5 litres of oil in it in 6-8 weeks