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7*RC is setting time to 0.1% residual error for a first order low pass. Can be a meaningful parameter, but I never found it designated "response time". Unfortunately, wpchan05 didn't manage to specify the requested parameter exactly. I don't see other candidates than tr(10%,90%), which is 2.2*RC, as said, or possibly group delay, which is 1.0*RC for f << fg.