Plots and tables detailing the wavelength sensitivity for each STIS mode can be found in Chapter 13 of the STIS Instrument Handbook.
Sensitivity degradation is typically less than 2% per year, as seen in plots of sensitivity vs. time below. This time dependence is incorporated in the STScI data-reduction pipeline for all STIS first-order configurations: it was implemented in May 2002 for the MAMAs (see ISR 2001-01) and on Nov 13, 2003 for the CCD. The reference files containing the most recent trends in the sensitivity degradation were delivered on 2017-Mar-10.
STIS sensitivity can be evaluated interactively using the web-based Exposure Time Calculators (ETCs). Count rates can also be determined manually from sensitivities, using definitions and formulae in chapter 6.2 of the STIS Instrument Handbook. Sensitivity may also be characterized in terms of limiting magnitudes for spectroscopic and imaging modes. The tables found in sections 4.1.2 (spectroscopic) and 5.1.2 (imaging) of the STIS Instrument Handbook list V magnitudes that yield S/N=10 for an A0V stellar input spectrum.