Please answer a question about the following article about Dutch language:

The Dutch written grammar has simplified over the past 100 years: cases are now mainly used for the pronouns, such as ik (I), mij, me (me), mijn (my), wie (who), wiens (whose: masculine or neuter singular), wier (whose: feminine singular; masculine, feminine or neuter plural). Nouns and adjectives are not case inflected (except for the genitive of proper nouns (names): -s, -'s or -'). In the spoken language cases and case inflections had already gradually disappeared from a much earlier date on (probably the 15th century) as in many continental West Germanic dialects.

Over what span of time have the rules for written Dutch become less complicated?
100 years