If it`s a glyphosate based weedkiller, such as Roundup or Tumbleweed, it`s safe now.
If it`s an ammonium sulphamate based one such as Deeproot, leave it for another 6 or 7 weeks.
If it`s sodium chlorate based, not until next spring at the earliest - may be longer depending on the concentration used.
If it`s one of the ones, such as Weedex, used for keeping paths and drives clear for long periods, up to 3 years.
Look on the packet and see what the active ingredient is and act accordingly