I don't know much about silkies yet, but I would think it would depend on food, care, and living conditions and breeding.. as that will determine how long they will live.. I breed Fancy Rats. and the general rule is they can live up to 6 years with good care, but some only live up to 2-3 because of the care, (eg.. spoilage, in food like candy, sugar, and etc) also breeding for longevity. Not that this is the same for birds, but we found if we bred our rats at an older age, the offspring lives longer and therefor reproduces this longer life span on to babies.
but Silkies may be different
I would thing an average lifespan for any bird is up in the 10++..since most birds I've met have lived that long, and I can't see it being any different for silkies..but again I'm not that knowledge in silkies yet.