Epson engineered the 157 Vivid Light Magenta Ink Cartridge for the Epson R3000 Printer. Epson utilized their UltraChrome K3 technology to produce deeper, richer tones.
With fine art printing in the front of their minds, Epson developed vivid light magenta to bring out the true colors of something as delicate as a flame. Epson's advanced magenta pigments bring to life the browns and the reds of your prints. Control your prints with the Epson advanced tones.
It really depends on the image. Light Magenta is one of the ones I change the most. I'd say 75 13x19 with normal saturation and color balance. More saturation can be dramatically less - 40-50. But I do buy 2 for every yellow.