Hi daimurph, welcome to A4a :)
definitely spray twice. It takes about two weeks for sprayed foliage to start showing die back and like tim says, you might see resprouting after a month. After the second spraying, wait a day or so then you can put young plants directly into the soil at the required spacing (e.g. courgettes, beans, squash, toms) - clear a space around the planting hole if you need to. The plants will thrive whilst any remaining weeds die off - I just leave what's on the ground as a mulch, to help conserve moisture during the summer.* I don't dig, but if you did you could combine this with saddad's green manure tip.
NB this won't work with delicate plants like lettuce or fussy root veg, but fast and gutsy growers will love it.
*if you have lots of slugs, rake it up and compost it once it's died back