Does your floor have a lot of bounce in it? If so, go every 12". If not, go every 18". I think if you'd rather err on the side of caution, go every 12" no matter what. HOWEVER, these days, with the use of engineered joists, you can go 18", no problem. Soooo, it depends as you can see.
Just be sure your screws are long enough to penetrate the joists AT LEAST 3/4". If you split the sub-floor or are finding it difficult to screw down, consider pre-drilling. Pre-drill 1/2 to 2/3rds the 'thickness' of the screw's diameter.