The simple thing to do is give it a limit on the total number of states it can explore in its search.
If your goal is consistency, wall-clock time makes no sense. If I run 'make -j20', should the chess computer become vastly easier because the CPU is being used to compile, not search? Should 'nice -n 20 <chess app pid>' make the chess computer worse?
Should my computer thermal-throttling because it's a hot day make the chess computer worse, so chess is harder in winter?
If the goal is consistency, then wall-clock isn't the simple way to do it.