The rate should never go below 5%, no matter what. There needs to be a standard where you either sink or swim. Stop getting drunk off zero percent interest rates.
There is an economic concept of the Natural Rate of Interest. This is defined as the interest rate that is neither stimulating nor suppressing the economy. It is an under-studied concept and it is difficult to measure an economy with a single value. For example, will lower interest rates stimulate consumer demand, leading to higher prices? Or will low interest rates stimulate manufacturer borrowing, leading to increased supply, and thus lower prices? Or a combination of both with an unpredictable outcome depending on what entities actually are doing the borrowing?
That said, people still do try to estimate the natural rate of interest. As far as they can measure, the natural rate of interest has been under 5% for quite some time. Sticking to an arbitrary lower limit of 5% would be harmful the economy in the long run.
Model-based estimates of the natural rate of interest, or r-star, and related variables
www.newyorkfed.org