Torches in a 1x2 tunnel

Solution 1:

Monsters spawn on light level of 7 or below. Torches give light level 14. Light level decreases by one with each block traveled. So you can place torches like this:

T 13 12 11 10 9 8 8 9 10 11 12 13 T

So torches have to be maximum 12 blocks apart.

When monsters are spawned, only the bottom block light level matters, so it is more effective to put torches on the ground, rather than on the wall.

The light conditions for mob spawning do not depend on the depth you are at. This is a common misconception that comes from a Notch's tweet a long time ago. However, this behavior is currently not implemented in the game (source: The Source for 1.4.5).