I'm afraid that without more information this is impossible to answer with any degree of accuracy. If the property is in an area which is wet, hot, or humid the rate of decay will be different from an area which has cold, dry conditions. The type of insects and their concentration in the area would also have a bearing on how fast a small animal would decay. To add to this, the degree to which the house was 'sealed' from the outside environment would matter, as would the place in the house where the animal was found. For example, I know of incidents where a cat has been found mummified behind a chimney in a home, had been there for decades and had still not decayed completely, the dry heat from the fireplace having prevented this. In a weather environment considered to be temperate, a large animal such as a sheep or cow (or indeed a human) left in the open can decompose to bones in less than a year, if insect levels and weather allow. In sub tropical or tropical areas this rate of decay can be increased markedly, to perhaps only a few weeks. If the cat was found inside a house and the home was fairly well insulated from the outside world, and if the environment was cold, the poor creature could perhaps have been there since before the house was left to stand empty.