\documentclass{exam}

\usepackage{blindtext,xparse,listings,xcolor }

\ExplSyntaxOn
\keys_define:nn { diaa / answers }
{
	d .dim_set:N = \l__diaa_answers_distance_dim,
	t .dim_set:N = \l__diaa_answers_thickness_dim,
	c .tl_set:N  = \l__diaa_answers_color_tl,
	d .initial:n = 5mm,
	t .initial:n = 0.2pt,
	c .initial:n = .,
}

\NewDocumentCommand{\FillAnswerRules}{O{}}
{
	\group_begin:
	\par
	\keys_set:nn { diaa / answers } { #1 }
	\leaders \hbox:n
	{
		\makebox[\textwidth][s]{
			\color{\l__diaa_answers_color_tl}
			\vrule width 0pt height \l__diaa_answers_distance_dim % the distance
			\leaders\hrule width \l__diaa_answers_thickness_dim\hfill
		}
	}\vfill
	\clearpage
	\group_end:
}

\ExplSyntaxOff

\begin{document}

	\begin{questions}
		\question short question
		\FillAnswerRules

		\question \blindtext[3]
		\FillAnswerRules
	\end{questions}
\end{document}