After WW II, as the Germans were devising the Basic Law (the Constitution).....they simply noted that early elections could occur, but they never laid out the practical ways that a door could be opened and early national elections could be held. Oddly, what the Basic Law does say is that neither the Bundestag or Chancellor can call for a early election....ONLY the President of Germany can do so. The President can only take this action if a confidence vote occurs, and goes negative (plus after this no-confidence vote....the Chancellor is supposed to go and ask for the early election).
Historically, you can go to a couple of occasions in the 1920 and this chaotic period from 1929 to November 1932 to note periods when an election should have occurred but everyone kept a problematic government in place and just kept pretending it wasn't a big deal on a lack of confidence.
How many times has this occurred in German history since 1945? Three.
1972: SPD's position of authority was challenged. Chancellor Willy Brandt (SPD, and former mayor of Berlin) had hyped up Ostpolitik (normalization of relations between West and East Germany). The CDU figured that they could push a vote of no confidence, and a quick election in 1972 would slide their way.
1972 result? Brandt and the SPD hung on.....with one of the largest showings of voters ever in Germany (91-percent)....and won. One of the odd features of this election...just two years prior, the voter age had been pushed from twenty-one to eighteen, and this probably helped in some ways with Brandt and the campaign.
1982: The FDP would quit the SPD-FDP coalition and force an election in the spring of 1983. One of the interesting features of the timing for this....is that the Green Party was organizing and would be a force in the spring of 1983 election. Politically, this put the CDU and Kohl....in a great position as some SPD lost some of it's normal voters to the Green Party.
2005: After a significant defeat in North Rhine-Westphalia for the SPD, Chancellor Schroder cranked up a no-confidence situation. There in the spring of 2005....the CDU was figured to be 20-percent ahead of the SPD nationally, and figured in a campaign setting to easily beat Schroder.
It was a marginal win for the CDU (Merkel) with only one-point separating them from the SPD.
In today's environment? A early election?
It is mostly unlikely but left to five state elections (two this fall....most with eastern German states....and three in the spring). If the CDU and SPD were to lose the normal voting pattern, and the AfD were to take 20-percent in each of the five elections...it might trigger a confidence vote with the Chancellor and the expected election in October/November in 2017......might occur a couple of months prior. But this scenario would require a million refugees to cross over into Germany again for 2016, and a trend continuing for the first month or two in 2017. The odds aren't in favor of that type of situation.
Elections do occur, but it's not the norm.