DearCustomer thankyou for your question.
The lifespan of most guineas ranges from 6-7 years.
Symptoms of dying might be appearance of paralysis,movement reluctance.
There are other symptoms usually related to disease condition of some sort and I have listed a website with a chart below to help you.
If you feel this information is helpful please remember to click accept and leave a positive feedback.
Excellent websites below:
Guineapig: Most frequent diseases. Vet Dr. Tiger gives information and advice.
Guinea Pig Care
How to look after a Guinea Pig