Eight, apparently. https://en.wikipedia.org/wiki/Javanese_calendar
What on earth is a windu?????
Look at the Wikipedia article I linked above. Apparently, it's a cycle of eight years. The Javanese calendar has other such periods.
"in A windu" should be accepted too
"Are" in this context is supplemental & not necessary.
How many years are in a windu should also be accepted.
how many years in a windu . is ever day talk. the verb is taken for granted
also, to be correct formal speech would say : how many years are there in one windu
My wife is Indonesian and from Java and says she never uses the word "windu" nor do people she knows.