While a cut & dry number would be convenient, it will have far more to do with the type of life and maintenance the parts have seen. Shocks can easily hit the 100k mark. I just swapped a set of shocks on my old Tacoma that were ~100k old with very, very hard miles between the owners. I've got local customers that have 10+ year old OME suspensions on daily drivers (Jason Day for you local guys), can't beat that.
Just to give you an answer, at 100k I would start planning to replace.