You would have to adjust the way tocloft
constructs the LoF entry by modifying the margin lengths that are set. Below is a redefinition of \l@figure
from tocloft
to provide you with the necessary output:
\makeatletter
\renewcommand*{\l@figure}[2]{%
\ifnum \c@lofdepth >\z@
\vskip \cftbeforefigskip
{%\leftskip \cftfigindent\relax
\rightskip\z@%\rightskip \@tocrmarg
\parfillskip -\rightskip
\parindent\z@%\parindent \cftfigindent\relax
\@afterindenttrue
\interlinepenalty\@M
\leavevmode
\@tempdima \cftfignumwidth\relax
\let\@cftbsnum \cftfigpresnum
\let\@cftasnum \cftfigaftersnum
\let\@cftasnumb \cftfigaftersnumb
\null\nobreak%\advance\leftskip \@tempdima \null\nobreak\hskip -\leftskip
{\cftfigfont #1}\nobreak
\cftfigfillnum{#2}}%
\fi
}
\makeatother
The commented parts have been changed.

\documentclass{article}
\usepackage[ngerman]{babel}
\usepackage{graphicx}
\usepackage[titles]{tocloft}
\makeatletter
\renewcommand*{\l@figure}[2]{%
\ifnum \c@lofdepth >\z@
\vskip \cftbeforefigskip
{%\leftskip \cftfigindent\relax
\rightskip\z@%\rightskip \@tocrmarg
\parfillskip -\rightskip
\parindent\z@%\parindent \cftfigindent\relax
\@afterindenttrue
\interlinepenalty\@M
\leavevmode
\@tempdima \cftfignumwidth\relax
\let\@cftbsnum \cftfigpresnum
\let\@cftasnum \cftfigaftersnum
\let\@cftasnumb \cftfigaftersnumb
\null\nobreak%\advance\leftskip \@tempdima \null\nobreak\hskip -\leftskip
{\cftfigfont #1}\nobreak
\cftfigfillnum{#2}}%
\fi
}
\makeatother
\newlength{\mylen}
\renewcommand{\cftfigpresnum}{\figurename\enspace}
\renewcommand{\cftfigaftersnum}{:\enspace}
\settowidth{\mylen}{\cftfigpresnum\cftfigaftersnum}
\addtolength{\cftfignumwidth}{\mylen}
\begin{document}
\listoffigures
\begin{figure}
\centering
\includegraphics[height=3cm]{example-image-a}
\caption{this is just a caption for a test figure, we add some more text here so the caption will span several lines}
\end{figure}
\end{document}
Keeping the \rightskip \@tocrmarg
option leaves you with the following output:

You could also use etoolbox
to redefine \l@figure
, if needed. It won't be much cleaner.