Suppose it depends on the weed. The only thing I use pre emergent is for crab grass. And its WAY too early to put down pre-emergent for crab grass. I find it does not start coming up until June. If that is corn glutten you are putting down (orange), I have read its effective for about 5 weeks.
Scotts Ecosence Weed-B-Gone does a very good job on dandelions. Be generous with it. Its a lot cheaper if you have a pump sprayer and buy the concentrated WBG.
I find Clover very difficult to control. WBG hurts it but has a hard time getting rid of it.