How much physical strength is required to control a Cessna 172?
I will take this from a more general perspective.

A Cessna 172 is part of the CS-23 category (in EASA land, see the equivalent Part 23 for FAA land).

In the relevant document, point CS 23.143, we find the maximum forces a pilot might be required to exert on the control columns so that the aircraft can be certified.

As the Cessna 172 is certified, we then know that it respects these requirements, so the maximum forces required to pilot a Cessna 172 (and any other CS-23 aircraft) are:

\begin{array}{l|c|c|c|}
\textrm{Values in Newton applied} &amp; &amp; &amp; \\
\textrm{to the relevant control} &amp; \textrm{Pitch} &amp; \textrm{Roll} &amp; \textrm{Yaw}\\
\hline
\ \ \ \ \textrm{For temporary application –} &amp; &amp; &amp; \\
\textrm{Stick} &amp; 267 \textrm{ N} &amp; 133 \textrm{ N} &amp; \\
\textrm{Wheel (two hands on rim)} &amp; 334 \textrm{ N} &amp; 222 \textrm{ N} &amp; \\
\textrm{Wheel (one hand on rim)} &amp; 222 \textrm{ N} &amp; 111 \textrm{ N} &amp; \\
\textrm{Rudder pedal} &amp; &amp; &amp; 667 \textrm{ N} \\
\hline
\ \ \ \ \textrm{For prolonged application –} &amp; 44.5\textrm{ N} &amp; 22\textrm{ N} &amp; 89\textrm{ N} \\
\end{array}