It depends on your system spec.
I run a Diamond Viper v770 TNT2 card with 32 mb. Plugged into my PII 266 with 96mb it gives me back about 25 fps.
If you want faster rates try going to Sparse scenery airports, and get the best gameplay you can. After all, I would rather the plane flies well than look at all the pretty buildings. I often wind the graphics down until I have landed. Then put the grahics all the way up and taxi in.
I'm buying a new PC soon though !!