try every 10 days. my girl got to the point where she was refusing after 7 but was very obviously hungry by 14 so i kinda went with something in between. when she starts refusing after the 10 days then i'll go up to 14.
my girl is about 3 ft long as well but she's gonna be a little bigger soon cuz she's in shed now.
try offering at 10 days & see what happens.