Note that there is a mistake in your phrase. The word 'its' is wrong. It should be '..when it's over with'.... The apostrophe ' tells you that the word is a contraction, in this case the contraction of 'it is'. The word 'its' is a possessive - the neutral form of 'his' or 'her'.

Yes, its = it is in this sentence. Over with = finished. You could also say "finished with".