Horizontal line of custom length like an —

I defined a \threeemdash macro for use in bibliographies. A more general \xdash version accepts the dash length as optional argument.

\documentclass{article}

\newcommand*{\threeemdash}{\rule[0.5ex]{3em}{0.55pt}}

\newcommand*{\xdash}[1][3em]{\rule[0.5ex]{#1}{0.55pt}}

\begin{document}

Some text: \threeemdash

Some text: \xdash

Some text: \xdash[6em]

\end{document}

If you're specifically interested in duplicating the en-dash look (which differs from a typical horizontal \rule), you could use leaders.

\documentclass{article}
\newcommand{\varendash}[1][5pt]{%
  \makebox[#1]{\leaders\hbox{--}\hfill\kern0pt}%
}
\begin{document}
Some text: -- \par
Some text: \varendash \par \medskip

Some text: --- \par
Some text: \varendash[10pt] \par \medskip

Some text: {---}{---}{---}{---}{---} \par
Some text: \varendash[50pt]
\end{document}

The above MWE provides \varendash[<len>] which typesets an en-dash -- within the space <len> (default is 5pt). Since the standard en-dash (--) has width 5pt, and em-dash (---) has width 10pt, using multiples of 5pt works best.

You could also define \varemdash in a similar context:

\newcommand{\varemdash}[1][10pt]{%
  \makebox[#1]{\leaders\hbox{---}\hfill\kern0pt}%
}

if this is your default usage. However, two en-dashes yield one em-dash.

For more on leaders, see Want to fill line with repeating string. For have a line filled to the end of the text block (like \hrulefill), consider using the xhfill package.