I have 4 rescue dogs, all over 10 years old. The younger 2 like it, other 2 (bonded sisters & older than other 2) scared of it, I have to hold as use on them & they run and hide if I let go of their collar(s). Other 2, especially youngest one, can't get enough, wants daily and an hour or more if I keep using. Unsure how long she'd allow. Only me getting tired stops a session on her!