Like Saigi said, the effect lasts for 3 turns or a single successful quality increase step, whichever occurs first. This will let you use Great Strides, Innovation, Steady Hand, and Byregot's Blessing, for example, in that order. Great Strides may be the only buff that lasts a set duration or single use though, so I can see why that's confusing. Man would it be OP if it just lasted 3 turns though XD