Some people say you should wash your blankets at least once per year but for me that's too long to wait to wash blankets.
I wash my blankets every few weeks otherwise I don't feel good and clean when I get into bed.
I have other clean blankets and sheets ready to go so every few weeks I'll remove the blankets and sheets from my bed and I'll throw them in the washing machine.
I love to crawl into bed with nice, clean smelling blankets and sheets and I sleep much better when I have clean sheets and blankets.
I would say for the best hygiene you should wash your bed sheets and blankets at least once per month or every few weeks like I do.
You could go a year or 6 months without washing your blankets like some people do but that's not for me and I prefer to keep the blankets and sheets clean more often.