How long does it take for a sunflower seed to decompose?

Answer 1

It would depend on the condition of its environment.  In a cool dry place, 100-2000 years.  In a hot humid environment, a couple of weeks.

Answer 2

That depends on whether it is whole or not.

If it is whole, it is still alive, and will not decompose. Rather, it will sprout and start growing in the spring. If it's in a dark compost bin then the sprout will then likely die, at which point it will then start to decompose (and do so quite quickly at that point, as it's very soft).

If you crush the seeds then the seed inside will decompose pretty quickly (a month or two). The outer shell will take longer, but should still rot within a few months on a well balanced compost heap.