I think this is a subjective question. It really depends on each individual, their circumstances, and how they feel at the given time. Some would argue that until life becomes unbearable while others may say whenever. Personally, in my opinion (and speaking for myself), I would CTB if and when life becomes unbearable and that I could not afford to hold on anymore. These can include but aren't limited to: terminal illness, severe disability (physical and psychological), sudden drop in QoL such that I wouldn't want to keep living, and more.