what OC we are talking about? also at which temp they are running, temp is basically everything, if you can maintain them cool enough and not with crazy OC that require Vmod or whatever, they should last for a very long time
OC to 500 GH/s I can keep them cool
how much is almost that long time
one can not know for sure(because there are numerous other factors, like the quality of the materials used to build those asic), but it should be not so much different than gpu, you should look at the product life(or what it is called) for nvidia or amd graphic production