From European Commission (EU) Regulation No 1178/2011:
FCL.710 Class and type ratings — variants
(a) In order to extend his/her privileges to another variant of aircraft within one class or type rating, the pilot shall undertake differences or familiarisation training. In the case of variants within a type rating, the differences or familiarisation training shall include the relevant elements defined in the operational suitability data established in accordance with Part-21.
(b) If the variant has not been flown within a period of 2 years following the differences raining, further differences training or a proficiency check in that variant shall be required to maintain the privileges, except for types or variants within the single-engine piston and TMG class ratings.
(c) [...]