I bought my Classic 30, which has a Volvo Penta D2020 engine with saildrive, in Spring 2002. I regularly inspected the diaphragm, which is easy to access, from inside the boat, using a torch. I finally had it replaced in Spring 2023 when therer were fine cracks starting to show on the surface of the rubber. So that was 21 years without any problem. I should point out that the boat is in the water in Chichester Harbour all the time, except for about 6 weeks in early spring every 2 years. Rubber degradation is bound to be temperature sensitive, and the diaphragm in my case would be at sea water temperature, almost always below 20C, and usually much lower. Out of the water, even in the UK, it would have been exposed to significantly higher temperatures in summer, and in the Mediterranean temperatures overall much higher, and degradation of the rubber, and possibly metal corrosion, significantly faster. V-P do not specify the temperature in their 7-year replacement recommendation, and presumably have to cater for a worst case temperature scenario, plus a safety margin.
Concerning exhaust elbows, do replace it in good time. Mine looked fine on the ouside, but in June 2024 it failed, a hole appeared in the wall and the engine would not start. I had not replaced the elbow earlier as I naively thought well, if it fails I’ll just replace it. The problem is, when the elbow fails sea water can get drawn into nthe exhaust valves, ruining the valve seats. That happened in my case, and the cylinder head had to be removed and sent to a machine shop to be put right. Expensive, and delays in the queues for the marine engineer then the machine shop meant no boat for the whole of July and August.
One comment about a stainless steel exhaust elbow: I do not know if it happens in this case, but generally if two dissimilar metals are in moist contact, the less “noble” of the two will suffer accelerated corrosion – in this case the cast iron adjacent to the stainless steel might corrode. It might be ok in this case, but worth checking. I have not looked it up, there might even be something about it on this forum.