Note: You can also set monetise uploaded video by default from uploads default page. Here is the direct link to the page. If not, when you are uploading a new video, you can click on monetise and activate the Adsense monetisation for videos. Though, the fastest way to monetise all your previously non-monetize videos is form my videos page.
All you need to do is, go to your my videos page (Video manager) and select the videos which are not monetised or the one you want to monetise. Now from the drop down menu you can select monetise and this will monetise all selected videos. Here is a screenshot for better reference:
If your account is in good standing, your videos will be monetise instantly. This is what happens to my Youtube channel, but if it’s taking time in your case, do let me know. You can check out my Youtube channel over here. At this moment, I’m making close to $60/month from Youtube videos, which is good as a side passive income.