According to the information provided by treasurydirect.gov, currently the rates for any Treasury bill ranging from 4-52 weeks are all above 4% (expressed yearly).
Looking at the 10 year/3month inversion spread, the graph is currently in the negative, meaning that the rates for short-term fixed income investment options are more favorable than long-term fixed income options. A negative inversion graph is reflected in the 4%+ rates for the most recent Treasury bills.
Therefore, I'm assuming 4%+ rate is currently good for Treasury bills.
More generally, I am wondering how can one determine at what percentages to draw the line between "poor", "average", and "good" investment rates, given that these rates fluctuate with the economic cycle?