And that is when? I have heard knowledgeable people talk about 10 years, but I have noticed that most people never change them, or the most conscious ones, only after twice that time.
What is your opinion, about the right time to have them changed?
Good question! Yes, the experts will say 10 years (esp in tropics) others say longer is OK, but as mentioned recently on another thread many, many boats are still sailing around with 20, 25 year old SS wire rigging and the rigs are still upright.
And like Tommays says, the service the boat's had over it's lifetime, the sizing and choice of fittings etc etc will have an influence.
As to the OPs question here, If he's worried about changing a stay, presumably because there's a crack in a swage, or it's developing a loose strand, or whatever... if one of your stays/shrouds is making you nervous then I'd say the rest of the rig is not far behind.
We replaced all our standing rigging last year - instigated by installation of a furler (well, may as well replace the forestay while we're at it....) which led to changing the rest at at the same time. It all LOOKED fine, may well have been 1984 original, but now we KNOW it's all new and not a concern for some time to come. btw - total cost (ex furler) ~ $1400... peace of mind? Priceless