The word you are looking for is "demi-decade".
Somebody sent me a message saying that people likely needed a citation to a dictionary or the like to validate my response in some unspecified way. I'm not sure I understand that, as the imminent good sense of the response seems to me to be fairly self-evident.
Nevertheless, if any of you feels the need for additional "authority" from Captain Obvious, please consult the English language dictionary of your choice to see that "demi" means half (as in "demisemiquaver", from the Latin "dimidius", which means to be "cut in half/through the middle"), and "decade" means ten years.
You do the math...