The best way to determine proper exposure is to test. This is especially important with paper. In the pinhole camera I use the most with paper negatives, the fastest black and white paper exposes perfectly in 30 seconds in bright sun; the slowest paper I've found exposes perfectly in 5 minutes. This is more than a 4 stop difference.

I use a light meter set to read EV. Bright sunlight is EV 15.5 at 100 ISO. If a scene meters EV 13.5, then expose for 2 stops more. For example, the 30-second paper would need to be exposed 2 stops or 4 times longer or for 120 seconds.

Here is the most lucide explanation of EV that I've found. It is for lens photography, not pinhole; but explains the principles well.
http://www.chem.helsinki.fi/~toomas/photo/ev.html

The same idea works with film. Even though all films' ISO ratings are consistent for lens photography, the reciprocity failure of films can vary widely, so testing films is valuable as well.