You don't provide quite enough information such as if you're talking about text or image ads. But here's how things work for text ads:
1. You create a new ad (editing an ad is creating a new ad).
2. The system does some automation to check if the ad follows policies. If it doesn't find anything unusual, it can start immediately. That doesn't mean that it won't ever be manually checked. A certain small percentage of ads will be for quality control.
3. If the system finds some issues, the ad is put into the queue to be manually checked by a human. There are a few different levels here, maybe two or three, and each level the ad is passed to will delay your ad being active until it is approved. The volume of ads they have to go through will also affect this delay.
If you experience delays of a few days, my guess is you are doing things that are on the fence. Maybe it's your business model, maybe your niche. For instance, I have a chiropractor client and often use a phrase in the ad that says you don't need drugs, because it works. That triggers a review for which I ask for an exception (he doesn't sell drugs) and gets approved. I don't track it but I think it's within a day or two.
If you have image ads, all such ads are manually reviewed by a human. If the queue is long, it will take longer. If there are issues with your ad, it will take even longer.
Occasionally, I've seen ads stuck in the review process for a few weeks. Seems they can fall through the cracks, maybe the reviewer forgot to flip the switch to on or something. I send an email and this always is resolved within hours but this hasn't happened in a long time because I create ads that I know will pass all tests.