Well it probably depends on your intention, how hard you tried to convey your message, your past criminal records etc etc but 15 months is unlikely... very unlikely if its your first time.
these are the punishments for denying the holocaust according to the wikipage.
DateNameCountrySentence
(various)
Jean-Marie Le PenFrance, Germanyfines
[55]
Feb. 27, 1998
Roger GaraudyFranceimprisonment (suspended), ₣240,000 fine
[56]
Jul. 21, 1998
Jürgen GrafSwitzerland15 months' imprisonment (fled Switzerland to avoid sentence)
[57]
Jul. 21, 1998Gerhard FörsterSwitzerland12 months' imprisonment,
disgorgement[58]
April 8, 1999
Fredrick TöbenAustralia7 months' imprisonment Mannheim, Germany - retrial - 2011 indefinitely stayed by judge Dr Meinerzhagen. October 1 - November 19, 2008, London, extradition to Mannheim, Germany, on European Arrest Warrant issued by Germany, failed. August 15 - November 12, 2009, Adelaide, Australia - for contempt of court because he refused to stop questioning the Holocaust's 3 basics: 6 million, systematic state extermination, gas chambers as murder weapon.
May 27, 1999
Jean PlantinFrance6 months' imprisonment (suspended), fine, damages
[59]
Apr. 11, 2000
Gaston-Armand AmaudruzSwitzerland1 year imprisonment, damages
[60]
Feb. 20, 2006
David IrvingAustria1 year imprisonment
[61]
Mar. 15, 2006
Germar RudolfGermany2½ years' imprisonment
[62]
Oct. 3, 2006
Robert FaurissonFrance€7,500 fine, 3 months' probation
[63]
Feb. 15, 2007
Ernst ZündelGermany5 years' imprisonment
[64]
Jan. 14, 2008Wolfgang FröhlichAustria6 years' imprisonment (third offence)
[65]
Jan. 15, 2008
Sylvia StolzGermany3½ years' imprisonment
[66]
Mar. 11, 2009
Horst MahlerGermany5 years' imprisonment
[67]
Oct. 23, 2009Dirk ZimmermanGermany9 months' imprisonment
[68]
Oct. 27, 2009
Richard WilliamsonGermany€12,000 fine
[69] [later overturned]
Jan. 31, 2013Gyorgy NagyHungary18-month suspended jail sentence
[70]