Write code to limit time for video showing

We have developed a website funfantasydates.com. The site presents videos uploaded to the webpage by video contestants. The video are open to members of the website for full viewing but we wish to set a limit of video viewing to 30 seconds by non-registered members that are surfing the site. Non-members viewing the videos should be notified after 30 seconds of viewing that if they wish to see the full videos they must register as members of funfantasydates. Currently there exist only 3 categories of paid memberships, but we also wish to include a free membership on the registration page which offers the registrant the ability to view the full videos by simply registering for a free membership that does not provide any other benefit to the member. Clearly we are trying to build a database of visitors to the site. Paid members also receive additional benefits including monthly credits that can be applied to a monthly lottery type draw, but free members will not receive any credits.