Half a century or five decades
That's called a millenium.
The simple interest over a period of five years is $463.70
That depends on whether the 17-year period includes three, four or five leap year days. The 17-year period that ended at the end of 2011 included four leap years and totaled 536,457,600 seconds. The 17-year period that began at the beginning of 2012 includes five leap years and totals 536,544,000 seconds. The 17-year period that begins January 1, 2097 includes three leap years and totals 536,371,200 seconds.
It COULD be called a Gigayear, but in practice it is simply called "a billion years".
a five-year period is a quinquennium
What funds are available for new obligations for a period of five years
The Gram Panchayat functions for a period five years.
A period of seven years is called a septennium.
There is no widely used term for such a period.
generation
sevennenium
100 years is a "century."
A synonym for every five years is quinquennial: as an adjective describes something that happens every five years or that lasts for five years; the noun means the fifth anniversary or a period of five years.
It's just five years, but you could say "Half a decade" if you want to be fancy.
a silver jubilee
A golden jubilee