How long skull fracture heal in child 15 months old?

Fracture healing. Children heal fractures remarkably quickly. As long as the fracture wasn't complicated (i.e. Markedly comminuted, or open, or infected etc) then it should be almost completely healed within 4 - 6 weeks.
Depends on the type. Linear, depressed, diastatic or basilar? Some types take 3-6 mos. To heal completely. See http://www.Childrenshospital.Org/clinicalservices/site2020/mainpages2020p4.Html. Traumatic brain injury warrants a referral to your state health division's early intervention program for periodic developmental assessments & any needed therapy. Safety-proof your home to prevent recurrence. Hope all goes well.