This varies from plan-to-plan depending on the administrator of the 529, how you are disbursing the assets, and if you've done so before. Looking at Bright Start in Illinois (not sure which plan you're using), it can take between five and ten days or longer depending on if you need to make an address change or otherwise update your account. From their program disclosure:
If timing is a concern, contact the plan in advance to make sure you know how long it might take to withdraw the money you need. You should also ask what paperwork they might require, if any, just to be prepared.
Brian Boswell VP, Research & Development
This information does not constitute tax advice and is provided for informational purposes only. Please consult your tax advisor, financial advisor, local taxing authority, and/or plan provider or sponsor for more information.