Ammonal will degrade but it seems that the rate is heavily influenced by environmental factors. As used by the British during WW1, it was sealed in tins or rubberised bags to stop the ammonal soaking up moisture which also meant it was protected from things that would degrade it.
There's a small but interesting article here about removal of such ammonal caches in 1998: -
http://www.durandgroup.org.uk/our_history.htm
The main site is here (no "home" button on the above link),
http://www.durandgroup.org.uk/
They do research and investigation of, in their words, "military-related subterranean features"