"Over" має багато значень, у даному випадку:
"Оver" preposition (DURING): during a period of time, or while doing something:
She made a lot of changes over the past six months.
Can we discuss this over lunch? (http://dictionary.cambridge.org/dictionary/english/over)
"decade" in British English
decade noun [ C ] B2 a period of ten years, especially a period such as 2010 to 2019
"decade" in American English
decade noun [ C ] a period of ten years:
"The economy is growing at its fastest rate this decade."
Decades means a long time:
"They’re enjoying new popularity after decades of neglect." (http://dictionary.cambridge.org/dictionary/english/decade)