We don't currently have a time limit, but do have an impression limit - the frequency parameter. If you want to fire one popunder once for every 5 pages loaded, you can use something like:
http://traffdaq.com/delivery/pu/1/?frequency=5
("1" would be replaced with your user ID. But, better get the code from the Get Code section)
For most users, a frequency of 5 would be more than acceptable. I hope this helps.
P.S.: I will consider adding a time limit as well. For admins that are continuously browsing the site, it can get pretty annoying. But a high frequency will do it temporary.