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If your visa is a multiple-entry visa with a validity period of at least six months, you can apply for a new visa up to six months before it expires (any time after 19 April 2019). Otherwise, you must apply within three months of your travel date (any time after 1 August 2019). This is specified in the Schengen Visa Code at Article 9(1):
CHAPTER II
Application
Article 9
Practical modalities for lodging an application
1. Applications shall be lodged no more than three months before the start of the intended visit. Holders of a multiple-entry visa may lodge the application before the expiry of the visa valid for a period of at least six months.