JW,Rusty Burlew, www.honeybeesuite.com, suggests anytime after the winter soltice.
I use 6 weeks before maples as a guideline. I put patties in my nucs on first of Jan. and open fed sub all winter. Maples came early this year, so only a four week head srart. Virginia has a short but intense flow from the end of April till mid June. I want my hives very strong by the begining of April to take advantage of this flow. What I do may not work in other areas nor be a good idea for those who are concerned about swarming.