After a Juvederm injection, how long until you can lay face down for a massage?

You should ask your injector, as they may have different guidelines. Generally, you should wait at least 24 hours or longer, depending on your treatment.  

I always advise my patients to wait at least 72 hours before adding any compression to the area. The guidelines can vary with area injected. Speak with your injector to answer the question specific to you,

You do not mention where the Juvederm was injected.  In general, I ask patients to wait 2 weeks after filler injections before having any other procedures such as facials, chemical peels or laser treatments to ensure any bruising or swelling has subsided.  This would also apply to lying face down for a massage.

Thanks for your question. Most nurse injectors recommend that you wait 24 hours before getting a massage, so please consult with your nurse injector for the proper instructions.

Each injector has his or her guidelines regarding post treatment care. Most injectors would recommend that you would wait until the next day before you have the massage. Consult with your injector as he or she knows the specifics about the treatment and you.

I tell my patients that the Juvederm will respond to pressure with some
movement for 72 hours after injection.