Change some coding to my existing site.

Hello, I already have a website that features videos, the videos are usually 25 minutes long. We have google text ads and banners in our website, i want you to change some coding in my site so that the google ad will hide after our desired time( like 2 minute or 4 minute) and again appear after our desired time like 30 seconds or 60 seconds. The thing is here, we want the ads codes to be refreshed when it appears again and again. Basically we want the ad code appear and disappear after certain time we want and again to republish there as a new page opened code.