After a facelift and neck lift how long do you recommend wearing a chin strap?

Best to ask your operative chosen surgeon. In my practice only a few days but each of us has our own opinions... 

My patients undergo a dressing change on post-op day 1, and the bulkier dressing is replaced with the facial garment/chin support that you're referring to. I have my patients wear that for 1 week, being able to remove it for showers and for short periods of time during the day. Definitely wear it while sleeping. Sutures are removed at 7-10 days and the garment is discontinued. I hope that helped. 

In our practice, we recommend it for 12 hours a day for 7 days.  Each surgeon has his or her own protocol that should be followed by their patients, since surgeons' techniques and surgeries are not all the same.