In "The forgotten soldier" by Guy Sajer, he states that they generally had a few days off from time to time in the city closest to the frontline.

Of course it depended also on the situation and when he is encircled with his unit in a Russian city (I don't remember the name) there is no real rest from the fighting and most of their waking time is used manning the lines and preparing for the next Russian assault as there is nowhere to go and even if there was they had no troops to spare.

Leaves are also becoming less and less common as the situation degrades for the Wehrmacht as sending troops on leaves means they are not fighting the Russians.

So basically it was highly dependant on the situation. As long as the situation was balanced / in favor of your army leaves allow your soldiers to remain relatively fresh. But it only is possible as long as you have enough troops to rotate them, when you begin to be in great difficulty leaves tend to disappear.

Another good source for the German perspective is Soldat by Siegfried Knappe. He was in the German Army throughout WWII and mostly stationed on the Eastern Front. Essentially, the Germans were given leave until the war started to fall apart for them.