Depends on the weedkiller Steve. Read the instructions which should make it clear how long from applying to planting anything. If you are concerned take a small sample of the soil you've treated and try to grow something very simple on it like cress, if that doesn't grow then nothing else is likely to either!

Usually, any weedkiller which is supposed to work in the ground rather than 'through the green', (meaning you apply directly to the growing leaves) has a residual effect in the soil, so nothing will grow there for a while. Pathclear and Bayer Path Control would be examples of that.

If you use glyphosate it works through being absorbed through the leaves so the soil is unaffected. It doesn't prevent new weeds germinating though. anything that does that will make the soil unsuitable for growing anything else for at least six months.