My 125i has 4000kms on it now
The fuel consumption has improved since I first got it
The question is a bit like 'how long is a piece of string'
It will vary depending on the driver, how its driven and the road conditions.
If I drove it like a 'granny' and had it in 'eco' mode you could expect to get over the 500km out of a tank (6lts per 100km). It only has a small tank.
Let's face it, you bought a 125i, it is a pocket rocket, and not having fun in it would not be doing it justice. Drive it and get the 'sheer driving pleasure' out of it