There is no good answer for this. Since they are not designed for this application and environmental conditions are different at each place the ties are being used, we cannot accurately predict the life span of natural ties used outdoors. For these applications, black or stainless steel ties are recommended.
The Nylon 66 material carries a cold temp. rating of 32 degrees, but cable ties are highly susceptible to cold and dry weather conditions and may break or crack. When the nylon begins to lose moisture it will begin to crack and break.
Gardner Bender FAQ 