When Thomas Gilbert Jr. received a 30-year sentence in September for killing his father over a money dispute, it ended a four-year-long case that sent a chilling warning to any parent who ever considered giving money to an adult child.

Mr. Gilbert, the son of a Manhattan hedge fund manager, was raised with a silver-spoon lifestyle, attending the elite Buckley School for boys in Manhattan, the exclusive Deerfield Academy in Deerfield, Mass., and Princeton University, but he had trouble holding down a job after graduation. So his parents gave him a monthly allowance, in addition to covering the $2,400-a-month rent on his apartment in Manhattan’s Chelsea neighborhood. When his father cut the allowance, an outraged Mr. Gilbert, then 30, took a gun and fired it into his father’s head at point-blank range.

“You want to support your child, but if your child is just serially not self-sustaining, what do you do?” said Christina Baltz, partner in the private client and tax team at Withers LLP. “It’s a real dilemma.”

While the Gilbert case is an extreme example, it speaks to a common dilemma for parents with money to spare: When and how much should they give to an adult child who comes asking for money — especially one who is able-bodied and well-educated? How long should any financial help last? And should it be a gift, loan or advance on an inheritance?