If you use Roundup or equivalent (do not use the long-acting product with a pre-emergent). Then you can reseed three days after the treatment. Do check the label for your specific product for any directions that differ for the reseeding schedule.
Some specific treatment limits: First some lawns will require two treatments with Roundup before reseeding (or planting sod). So wait 7 days after the first application to identify any green (growing) spots and then retreat. After all existing lawn grasses and weeds have been treated, you can rake the area and then put down sod or grass seed 3 days after application.
Second: for best results the plants treated with Roundup need to get rain or irrigation a few days before the application and 2-3 days after the application. (This is true if the soil is dry when you are getting ready to plant.)
I hope this helps.
Cindy Wells
(the labels for several Roundup products are here:
http://www.scotts.com/smg/catalog/productCategorySubSelf.jsp?itemId=cat50096&navAction=jump . Click on product information and then the label.)