And here is where we start to get into just how long Shredder has been around. Although it's not ever established, in my head I put him about two years before when the TMNT show up.
The reasoning for this is simple. He warped through a few moments before the TMNT did. That would mean he could have dropped in at any point earlier on the timeline. Hours, days, months... or years.
Of course, a more cynical person would point out that he could have warped in just about anywhere in space/time. If his point in space was fixed, but not his point in time, he could have dropped into deep space, as the planet would be just about anywhere on it's orbit around the sun (and, similarly, the sun would not necessarily be in the same space in the universe as it was when he went into the portal).
But that's a heavy load of science talk for a simple web-comic. I'll leave those kinds of discussions to XKCD.
Also, this comic is interesting for it's layout. I usually do panels stacked on top of one another, but it's rare for me to have one panel (Darkmoon's) rest entirely inside another. I was feeling playful, apparently.
 
    

When evil spreads across the land, and darkness rises and the monsters roam. When the creatures of the night make beautiful music, and the things that go bump in the night go bump with greater enthusiasm. When the world is in peril and is in need of a hero...

These guys are, sadly, the best the world can hope for. These are the adventures of the heroes of CVRPG. They mean well, they try hard, and occasionally they do the impossible...

They actually do something heroic.