Adobe Premiere Pro CS4 - Looping a clip
Is the watermark a video or a still image? If it's a still you can usually just increase its duration. 

Otherwise, copying and pasting is the best way to go about it. Copy and paste it 10 times, then select the 10 clips, copy and paste that 10 times, then select the 100 clips, copy and paste that etc. until you get the desired duration.