The longer the distance, the more difficult it is to accurately arrive at a correct judgement.
The best way of learning is to get to know somewhere near home, and measure out by footsteps (typically about 1 metre/yard for a normal footstep).
Obviously this can vary enormously between individuals and using a tape can tell you what your footstep length is.
With this info. to hand, then step out to objects at varying distances, and make a written note of these. When out in the field, with camera at the ready, you can then picture the distance of a given point from your notes. The more you do this, the more accurate your results will be, until your judgement becomes second nature.
However, at longer lengths, one's ability decreases, and the old method of judging one third of the way into a scene is arguably the best option. Also, the longer the distance away, the need for accuracy reduces.
Hope this helps