Donaudampfschifffahrtselektrizitätenhauptbetriebswerkbauunterbeamtengesellschaft

80 letters
Translation - Association for Subordinate Officials of the Head Office Management of the Danube Steamboat Electrical Services
When to use it – after a dozen schnapps with the aforementioned captain.

Freundschaftsbezeigungen

24 letters
Translation: demonstrations of friendship
When to use it? Frequently, at Oktoberfest.

Kraftfahrzeug-Haftpflichtversicherung

36 letters 
Translation - motor vehicle liability insurance
When to use it? While renting your Mercedes at the Avis desk

Rechtsschutzversicherungsgesellschaften

39 letters
Translation - "insurance companies providing legal protection."
When to use it? While booking a walking tour of the Black Forest – just in case the local operator goes bust

Massenkommunikationsdienstleistungsunternehmen

46 letters
Translation - companies providing mass communications services.
When to use it? While looking for a biergarten in which to watch the big football game.

Siebenhundertsiebenundsiebzigtausendsiebenhundertsiebenundsiebzig

65 letters
Translation - 777,777
When to use it? When predicting to a local bookmaker how many goals Brazil will concede in their next match with Germany.

Sozialversicherungsfachangestelltenauszubildender

49 letters 
Translation: a trainee assistant social insurance broker 
When to use it? While discussing Bayern Munich's footballing dominance, for example: "This year's Bundesliga title race was as boring as a Sozialversicherungsfachangestelltenauszubildender".

Rhababerbarbarabarbarbarenbartbarbierbierbarbärbel

50 letters
Translation: the Barbie of the bar where the beer of the beard barber for the barbarians of Rhubarb’s Barbara’s bar is sold (see video below for an explanation)
When to use it? Don't.

http://www.telegraph.co.uk/travel/destinations/europe/germany/10966002/Long-German-words-a-phrasebook-for-show-offs.html