How to bend more than 90 degree?

Option 1

You could change your bend right=90 to out=-45, in=45.

Option 2

You could use a Bezier curve. Adjust the 5 to the value you like.

\draw[red](b.east) .. controls +(5,0) and +(5,0) .. (a.east);

Are you looking for the looseness Option?

\documentclass[margin=5pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{positioning}
\begin{document}
\begin{tikzpicture}
  \node(a){test};
  \node(b)[below=5cm of a]{test2};
  \draw[red,bend right=90,
    looseness=3
  ](b.east)to(a.east);
\end{tikzpicture}
\end{document}

Or with

\draw[red,out=-20, in=20,
  looseness=3
](b.east)to(a.east);

You can use distance:

\draw[red](b.east)to[bend right=90,distance=7cm](a.east);

Code:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{positioning}
\begin{document}
\begin{tikzpicture}
\node(a){test};
\node(b)[below=5cm of a]{test2};
\draw[red](b.east)to[bend right=90,distance=7cm](a.east);
\end{tikzpicture}
\end{document}

Or you can put the controls straight in the options of to operation.

\draw[red](b.east)to[controls=+(0:7) and +(0:7)](a.east);

Code:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{positioning}
\begin{document}
\begin{tikzpicture}
\node(a){test};
\node(b)[below=5cm of a]{test2};
\draw[red](b.east)to[controls=+(0:7) and +(0:7)](a.east);
\end{tikzpicture}
\end{document}