Not all figures were wild estimations. There are an estimated 200 to 400 billion stars in our galaxy. This number has been around for decades (may prove to be wrong later, but this is the range astronomers generally consider). More recent figures (based on our planet-finding via star wobbling/periodic dimming) suggest at least several billion Earthlike worlds alone. Systems so far discovered have at least one planet, on average. So I figured (a very conservative estimate) one planet per star.
Even if our galaxy has "only" 250 billion stars, that's a total of 500 billion stars/planets just in our galaxy alone, again, not so wild an estimate, given what we've been finding out there. With two galaxies: we now have a trillion celestial bodies to explode.
Each detonation I'm calling an "action." So we have at least a trillion actions to be performed, within a 5-year period. 5 years is roughly 158 million seconds. The math here is straightforward for #actions/second.
Now come the wild estimations. Each travel period from star system to star system, from planet to planet, I'm also calling an "action", and so now there would be a ratio of time-spent-traveling to time-spent-blowing-things-up. You can make this ratio whatever you want, but let's say this raises the number of "actions" needing to be done to 10 trillion. Again, the math is straightforward, involving a mix of conservative figures with liberal/wild estimates.
Thing is, even if I'm off by a factor of 1000 (in one direction), Surfer is till traveling to and blowing up dozens of stars/planets per second: still a feat to make any other herald-level character (*coughthorcough*) blanche. If I'm off by 1000 in the other direction (he's destroying millions/second), then even Galactus would go uh-oh...
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