The Huffington Post recently teamed up with online polling firm YouGov to survey U.S. adults on the performance and receipt of "helicopter" parenting behaviors. Results are reported here, with the article containing links to additional study details. (For some of the measures of assistance-receipt, results focus on individuals 33 years-old and younger, rather than the full adult population.)
As the article discusses, it is hard to define helicopter parenting with precision, as after all, "...one person’s loving guidance is another person’s overbearing supervision." One marker the article offers is "whether a parent does something for a child that is developmentally inappropriate." For example, by intervening when their child has made a mistake in a relatively low-stakes situation, are parents depriving the child of an opportunity to learn from the consequences? Are parents providing financial assistance only for their child's basic needs or for everything, including entertainment? Asking about different kinds of parenting, and their consequences for grown children, should help refine our understanding of optimal parental involvement.
To learn about some studies I have conducted in this area, see this page.