I’d say it takes about 4 to 6 weeks; partly it depends on what you’re planting. What I did, a couple of years ago, is start by smothering and mulching with compost (rather than wood chips, for example): after a few weeks, you can plant annuals into the mulch, and after a few more, shrubs and perennials into the old sod. If you find, digging down, that you hit yellowed sod, you can dig out the bit in the hole you’re working in, place your plant, mulch it, and keep going. You don’t want to uncover sod, is all, because it revives with alacrity. Keeping the ground covered is the trick.