My experience is that is takes about two weeks for the sprayed area to turn completely brown. However, there is no doubt that the plants have already started to die. If you plan to disc or till the plot before you plant your clover you will get weeds back anyway. I don' t care what you spray a plot with if you disc it weeds will come back. The seeds are there and they germinate upon discing and bringing to the surface. The only way I think you can get a weed free plot after discing is to spray it yourself with a weed specific herbicide or spray the plot with round-up, never disc it and frost seed the next spring. It may take a while to get a strong plot with frost seeding but eventually you can. Also, if you have seed drift from neighboring weeds or fields, weeds will come back. This is why it is so important to mow before weeds in your plot have a chance to re-seed themselves. This is my opinion but there are a lot of other people on here with more experience than I.