You should dry them until they are dry.

Seriously, if you look at the emulsion side of the negative as it is drying, you can see a distinct change in the surface as moisture evaporates. Roll film negatives dry from the outside in, and you can see the surface taking on a more dull appearance starting at the outside edges, and gradually progressing toward the center of the strip. Leave them hanging until the surface appearance is uniform.

If you are drying sheet film negatives, you will find that moisture will accumulate at the bottom edge (or bottom corner if the negative is not perfectly plumb). You should let them dry until that drop of moisture disappears completely.

Film emulsion expands very slightly when it is wet, and you want to make sure that it has dried completely before you handle the negatives. Putting film into sleeves before it is fully dry will cause (1) the negatives to stick to the sleeves and (2) scratches. Either will ruin the negatives.

The time required to dry depends on the temperature and humidity in the area where the negatives are hanging. You can speed up the drying process by passing warm, dry, filtered air over the negatives. In a heated drying cabinet with forced air flow, negatives will dry in less than an hour. If you just hang them in a room, it can easily take overnight.