system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do doctors determine what stage cancer is in and how long a patient has to live?
gpt
Stage one- cancer is small and localized.  
Two- has grown but still localized  
Three-  grown more and possibly spread to lymph nodes  
Four- has spread to other organs and areas of the body  
The time is always an estimate. Likely an average just based on other people at that stage, and how healthy they are otherwise.