An ecosystem is an area that can produce nutrients and sustain life.
Ecosystems can be any size, from the vast rainforest of the Amazon to a small woodland area or a tiny pond perhaps at the bottom of a garden. There are also different types of ecosystem. They vary from place to place around the world as well as between and within areas such as woodland, moorland, streams, farms and even cities. These variations are due to largely to the different physical conditions that exist in any particular location and they can be studied using a system approach.
Soil, sand, gravel, rocks, water, air and sunlight are all abiotic factors found in a desert.
No one set answer for this. Many variables determine how long this takes. Material used, climate the house is in, other aggravating factors... some decompose in a couple of years, while there are historical cabins in Antarctica which are virtually unchanged from their condition in the early 1900s.