How bad is it to keep wearing old contacts? What damage does it do to your eyes?

Keep on schedule. Contacts are designed to last for certain intervals of time which will be told to you by the ophthalmologist who fit them for you. This relates to the time to accumulate substances which will affect clarity, change comfort or culture organisms. There is some flexibiity beyond manufacturer recommendations related to environment and your tear cleaning ability. Best to keep on schedule.
Blindness. If you want to be blind, keep wearing your old contacts and you will get an infection.