This is a very common questions and for the best answer please read page 45 of the Applications Manual for Portable Magnetometers.

To give an idea, the rule of thumb is that 1 ton of steel will give 1nT at 100 ft. The distortion caused by the steel in the earth’s field falls off as the cube with distance and is linear with mass. Therefore, at 50 ft, 250 lbs will give 1nT, at 25 ft 30 lbs will give 1nT, at 12 ft 4 lbs will give 1nT. Cables and pipelines fall of at somewhat a different rate (inverse square) so can be seen further for a given mass.