SM4 is correct it is 15 sitting days, not calendar days, so depending on when the Senate is sitting, it could well be three months.
Go to the Parliament House website, go to the Senate and check out the "sitting pattern". It will explain which sitting days are counted and which are not, say senate Estimates may not be.
Or ring the Clerk of the Senate and you will get some very helpful advice. There are no secrets or tricks, it's called democracy in action.
If you want a dissallowance you will need to find a Senator who will sponsor it AND who can get the support in the house to pass it.
You can also play, with the Senators cooperation, "relays" on the 15 sitting day routine for a long time if you need time to get your act together.