I just watched the German World War II miniseries Generation War, and the two brothers serving in the Heer seemed to serve on the Eastern Front from the invasion until the end of the war.
How long did Wehrmacht deployments last?
Well, to quote Kipling: "There is no discharge in the war!" You didn't go on deployment in the Wehrmacht, you were entitled to, under the best of circumstances, two weeks of leave twice a year (for the Field Army/ combat troops) or once a year (for the Replacement Army/reserves, garrison personnel and other non-combat elements) respectively. Other than that, you served until you were either dead or physically/mentally incapable of performing both, front line(KV) and garrison(GV) duties.
There were, however, a lot of caveats: There was a certain amount of travel time calculated and added (two days from Russia to western Germany in 1941), but depending on the unpredictable nature of reasons for transportation delays, that time wasn't always enough. So depending on where you were from (leave was always to your home region where you had to "report in" once you arrived), where your unit was deployed, and how the transport situation was (overcrowded trains [transportation towards the front had priority over anything going in the other direction], no available ships, delays due to partisan activity and air raids), you might have considerably less time at home. you could also be recalled early, though that was a measure the regime tried to avoid at pretty much all costs, as it was seen as potentially damaging morale not only of the soldiers, but also on the "homefront".
extra vacation time of varying length was given for a host of reasons, among them:
all this, of course, operational situation permitting and at the discretion of you superiors - usually your company command.
as there was extra vacation time, there were also, of course, blanket cancellations of leave for all units for a fixed time (Urlaubssperre). Almost exclusively for operational reasons, this could result in entire army groups/corps' not getting any leave for well over a year. Oftentimes, this coincided with a general news blackout (Nachrichtensperre), which meant that on top of your leave and rest time being cancelled, there was also no mail to be sent or received.
That covers actual leave (Fronturlaub/Heimaturlaub). A different beast entirely was the time spent away from front line duty, either with the unit (refreshment/refitting/training/deployment to a different theater of operations etc.) or individually (assignment to a different unit, hospital and recuperation time, time-limited teaching assignments and so on).
Generally, leave was granted liberally and fairly (those who had been without leave the longest would be the next to go on leave), whenever the operational situation permitted it, chiefly for reasons of morale both at home and among the troops. If you didn't fall afoul of the system (revokation of leave was a popular form of non-judicial punishment), and were exceptionally unlucky with your unit/travel situation, you'd seldomly see less than three weeks of accumulated leave time per year, at least until the war started truly turning for Germany - around, say, 1942 or so. Please note that this last number is an educated guess - I've never come across a definitive number, it is my best estimate deduced from a knowledge of the various variables mentioned above and a lot of reading. In the end, it all very much depends on the unit and individual soldier in question. The 4th Panzer Division, just to provide an example, only saw 5 1/2 official rest days on the Eastern Front between June 1941 and March 1942, and the majority of their personnell hadn't been on leave since fall/winter of 1940. At times severe performance issues due to simple burn-out of even the most tried and tested troops were a common theme for combat commanders, especially on the Eastern Front.
Sources:
Lots of bits and pieces from all kinds of primary and secondary sources, but especially:
C. Hartmann: Wehrmacht im Ostkrieg K. Richer: Kavallerie der Wehrmacht M. van Creveld: Kampfkraft