I usually wait about two weeks. Glyphosate (Roundup) needs the leafy surface to be drawn into the plant and down into the roots to complete it's effectiveness. It works best on actively growing plants. If you till up the plants to early, they quit their growing process and the glyphosate doesn't have time to do it's job. I just sprayed last Friday and plan on tilling around the second weekend in May.
I like to roundup, wait a week,til-r-up, let it sit for three more weeks(longer if it's cool). I find some seeds get unearthed and start to sprout. Hit'er again with the roundup. wait a week, then plant.
You still have time to do it this way and you will be happy with what little weeds you'll have. Grass seems to be more of an issue for me unless I'm trying to get it to grow in the yard!