Really bad.
Your car senses air volume using a "HFM" - Hot Film Meter. This is a delicate piece of material, thinner than paper. Electrical current is run through it so it heats up, and the car determines the volume of air (automatically accounting for temperature) via the change in resistance (due to change in temperature) of the wire.
So with this extremely delicate, sensitive, and
expensive part in your intake tract, you should strongly take the advice that the air filter is a critical piece of protection that should not be left out.
In case I was too long-winded (as usual), I'll sum it up:
put the air filter back in as soon as possible!
HTH,
- Rob