How long is a piece of string?
Since we don't know exactly what resources you have, nor how microsoft measure these resources or which ones are most important in your particular setup it is impossible to give a meaningful figure. I've seen systems with 90% resources free which have been running really slowly, and others with 50% free with no noticable degradation.
The bottom line is that the resources figure applies to YOUR machine, and by monitoring the resource meter you can see when available resources are falling and should possibly think about closing down some of your programs.
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