It's not an actual limitation. Double-click on your matte to open it into the Viewer. In the Viewer's Duration column (upper left), enter a new number -- say, one hour (1000000). Now you can drag the piece in the timeline to a length of up to one hour.
That said, I don't recommend having such a large overlay. I'd keep it to 10 minutes apiece and paste it multiple times over your timeline. It's very awkward for rendering to have a one-hour matte on your video, and you'd be much more prone to mistakes doing it that way.
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