It depended on the part of the front and the stage of the war. No man's land could be anything from a few dozen yards' apart (for example the distance between Hammerhead Sap and Oblong Wood, from which positions the British and Germans, respectively, raided one another ahead of he Battle of the Somme outside Thiepval), to a couple of kilometers, depending on the terrain.

Both sides suffered a severe shortage of artillery ammunition early in the war, and even in 1916 the sides could only really blast the enemy at chosen points. So Verdun and the Somme were heavily shelled, worked right over, in certain positions such as The Mort Homme and Poizieres soldiers couldn't find any landmarks at all, and maintaining orientation and position was extremely difficult.

It was mostly too difficult to tidy up such areas, even to bury bodies, which is why more than half the dead on the Western Front are 'missing'. Amid this was wire, bricks and other detritus of war including horses and men shattered by the shelling, which until 1917 was mostly shrapnel.

This was a far cry from the many less 'strategic' parts of the front which were more orderly.

In early 1917 the Germans shifted back to the Siegfried Line (what the Allies called the Hindenburg Line), fresh new ground, which was then fought over again with increasing artillery forces available. This gradually smashed the ground again, and places like Vimy Ridge and much of Verdun still bear witness today to the endless holes and moon-like conditions that prevailed. By now trenches to be attacked could and would be completely destroyed, replaced with endless shell-holes that surviving soldiers would try to link with desperate digging.

There were never enough tanks for these to be common landmarks in No-man's land, but they were knocked out prodigiously where they were used, so they would certainly have been a feature of the battlefield in battles such as Cambrai and Amiens, but they would have been salvaged fairly soon afterwards for scrap, parts or repair.

The battles of 1918 largely took place on new territory, the Germans attacked out of the Hindenburg Line, driving the British back over the old Somme battlefield. There are many accounts of how shattered and desolate this area still was, completely destroyed, and it was an obstacle to the German advance.

The Allies then attacked after the exhaustion of this offensive, being successful in part because the Germans were no longer in long-established defence systems: indeed at the Battle of Amiens the trenched were quite minimal, and on one flank artillery was fired specifically to give attacking troops a covered position on their objective.

Follow up question: How did the area authorities feel about private citizens taking military equipment. I understand it wouldn't be common but a mechanically inclined citizen might think of restoration.