Great question. The quick ratio tells you how fast you are growing over time (could be m/m, w/w or d/d depending on what time frame you choose). How fast you grow strongly depends on the phase of growth you are in. So, there is really no magic number of what would be a healthy quick ratio. Also, regardless of how fast you are growing, if you are not retaining people well as you are growing, this is unsustainable. As such, I would look at the two dimensions together — retention and quick ratio — and form an informed option of whether you are growing sustainably.