The label on the weed n feed should have all the information you need. It may vary from product to product because the active ingredient or the formulation may be different. In general, I would say that reapplication after 2 weeks is too soon.
The best time to apply is when you first see weeds actively growing. Granular products should be applied after a rain when the grass is still wet. If no rain, water the lawn first then apply. Never apply weed n feed to your lawn when the temperture is excessive - again the label should have this information.