What are the risks of wearing headphones for a prolonged period of time?

Headphone risk: I assume you mean they are turned on to listen to music? if so then early deafness may be in your future. Or you may be run over by bike or car or truck. Wax may build up in your ears and you go deaf if you never take them off and wash ears. You could become allergic to the material that covers your ears and break out. May be more you can search pubmed for Earphones AND health risks.

Risks of headphones: Damage to hearing depends on duration, even reasonable volumes can damage delicate hair cells in the inner ear. Also, consider the social isolation that occurs with prolonged use of headphones. Moderation in duration and volume www.nlm.nih.gov/medlineplus/ency/patientinstructions/000495.htm www.nhs.uk/Livewell/hearing-problems/Pages/tips-to-protect-hearing.aspx