I've added some code at the end to suggest one way of automating this. Hopefully someone will come along and show you a better way to do this.
Nevertheless, here's something of a working solution. Please note that I can not promise that this will work completely bug free. For example, I'm not entirely sure how this will work for a chapter without enough space.
This answer was greatly inspired by
@MartinScharrer 's answer to How to define a figure size so that it consumes the rest of a page?. What I've added to his answer is a means of counting how many lines you can fit on the remainder of the page. I've also loaded the tikzpagenodes package: this greatly cuts down on the amount code that needs to be written.
\documentclass{book}
\usepackage[margin=0.5in]{geometry}
\usepackage{tikz}
\usepackage{tikzpagenodes}
\usetikzlibrary{calc}
\usepackage{lipsum}
\pgfmathsetmacro\ruledheight{0.5cm}
\newcommand{\ruledNotesAtEndOfChapter}{%%
\ifdim\dimexpr\textheight-\pagetotal\relax>1.5in\relax\vspace{1in}%%
\noindent
\begin{tikzpicture}[overlay,remember picture,y=\ruledheight]
\path let
\p0 = (0,0),
\p1 = (current page text area.south)
in
\pgfextra
\pgfmathsetmacro\imgheight{\y0-\y1}%
\pgfmathparse{int(\imgheight/\ruledheight)}
\edef\mynumberoflines{\pgfmathresult}
\foreach \y in {1,2,...,\mynumberoflines}
{
\draw (0,0) ++ (0,-\y) -- (\linewidth,-\y);
}
\endpgfextra
node[anchor=south west] at (0,-1) {\large\textbf{Notes:}};
\end{tikzpicture}%%
\fi}
\makeatletter
\let\stdchapter\chapter
\renewcommand*\chapter{%
\ifnum\value{chapter}>0\relax
\ruledNotesAtEndOfChapter
\ifodd\value{page}\relax\clearpage\ruledNotesAtEndOfChapter\fi
\fi
\@ifstar{\starchapter}{\@dblarg\nostarchapter}}
\newcommand*\starchapter[1]{\stdchapter*{#1}}
\def\nostarchapter[#1]#2{\stdchapter[{#1}]{#2}}
\makeatother
\begin{document}
\chapter{hello}
\lipsum[1-9]
\chapter{middle}
\lipsum[1-15]
\chapter{ciao}
\lipsum[1-15]
\ruledNotesAtEndOfChapter
\end{document}
You can change the line spacing by resetting the \ruledheight macro to something more suitable. Note that I set the y scale in this tikzpicture. I was having problems with getting the spacing to work correctly in the body of the picture itself.

Update on remaining space on the page
I've added a wrapper for the macro \ruledNotesAtEndOfChapter, namely:
\ifdim\dimexpr\textheight-\pagetotal\relax>1.5in\relax\vspace{1in}%%
.
.
.
\fi
This adds a little bit of space after the last line of text (by the amount in the \vspace command). But I also test for the remaining space on the page. If there isn't enough space, then no notes are drawn.
UPDATE on automating this
The redefinition of the \chapter command was plagiarized from Custom \chapter definition
Several points about the redefinition of \chapter
First, notice that I test for the chapter number. That's to prevent notes from showing up too early in your document.
Second, the last chapter will not have any notes; so, you'll have to enter the notes automatically for the last chapter.
Third, I've rewritten \chapter so that when a chapter ends on an odd page, the blank page is also used for notes.
\endofchapterrulednotesmacro, then I've got some ideas. But, I would still need to know how to handle situations where there are not enough space at the end of a chapter. – A.Ellett Sep 29 '13 at 15:14