One way to tell is if you have an oil pressure gauge. In my LT1, I know the oil is up to operating temperature when the idle pressure is down to 20psi, or when it's just under 40psi at highway speeds. When it's still cold, the pressure is much higher than this.
In my car I've noticed it takes about 15-20min to reach this point. Cars with large cooling systems, such as mine, will take longer for the oil to heat up also. Oil temp always lags behind coolant temp. With a small cooling system, such as on my wife's Civic, it will heat up faster, so thus the oil heats up faster too. I would say in a small car that the oil should be up to temperature in 10-15min instead of the 15-20 of a larger engine. Obviously wintertime adds a few extra minutes to these times.