How much longer will my 10 year old frame last?
The slightly sarcastic-sounding comment is correct, unfortunately.

Even if you used a high-end xray imaging system to carefully analyse the frame, the best you could say is "there are no obvious defects found". You might find major defects, in which case you'd probably recycle the frame rather than riding it, but if you don't you haven't really learned a huge amount - there might be defects just below your ability to detect them, or of a type that you can't detect. In general this is a class of problems summarised as "you can't prove a negative".

More practically, if you carefully inspect the bike for obvious cracks, dents and other major damage, and don't find any, it's probably safe to ride. I have bought and ridden bikes like that for years, generally until so many things are worn out that it's cheaper to buy a new bike than repair everything. That is not unusual with bicycles, barring crashes the frame is often the last part to fail, and the failure is often "it wore out" rather than "it broke". Threaded parts get too worn to hold whatever screws into them (bottom brackets), friction-fit parts get loose (like headset cups), parts that wear, wear to the point of being dangerous (dropouts).