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:

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?

Forum discussion at Google AdWords Help.