https://msdn.microsoft.com/en-us/library/ sk4bwd6h(v=vs.90).aspx

It doesn't say the default, but the default is 19 on asp.net, and it's most likely the same in MFC https://msdn.microsoft.com/en-us/library/ system.windows.forms.treeview.indent(v=vs.110).aspx but you can always just output the result of a GetIndent call to see.

Assuming 19px, that could be lowered to 14px or basically however low you can go without the folder icon overlapping with the +/- buttons that way we can see more nested folder levels before we have to horizontally scroll.