A Google AdWords Help thread has Google's Charvi explaining how long an AdWords ad can take to be approved.
Typically, new or modified ads are approved within 24 hours but it can take as long as 3 days.
The Google representative said:
All new ads are submitted to us for review. We reviewed these as quickly as we can - usually within 3 business days and most often within 24 hours! If your ad complies with our policies and guidelines, it should be approved automatically. Sometimes, we need a human to confirm that your ad can be served.
In addition, edits to ads may require approval, even small edits. Google says edits to ads will be "treated like a completely new ad." And when you edit an ad, all the statistics for that ad are reset to 0.
The tip offered?
To avoid any down-time in which your ad isn't running, create a new ad with the changes you need and pause the old one once the new one is approved!
Forum discussion at Google AdWords Help.