i don't listen to myself type

What is subjective time? I don't get it.

basically, it means that they experience time differently than us. a hundred thousand years passed in the blink of an eye, because the rate at which time passes is determined by the one experiencing it pass.

Then wouldn't all the Housepets! characters be dead, or do they just run on a different calendar?

separate timeline. because Pete experienced it as a hundred thousand years, but all the people of the court experienced it as just a few seconds, same as our universe.

How much time passed for Earth? If that much time passed on Earth wouldn't all the characters on Earth in this comic be dead?

I assume the few seconds that everyone but Pete experienced also passed on Earth.
Of course, I may be wrong.

It works like the Hyperbolic Time Chamber (or the Room of Spirit and Time) does in DBZ 1 year in the chamber is a day on earth. Its just an extreme version of that 100,000 years to Pete (and Heaven I would guess) was like a few seconds to everyone else.

can I think of it as a back to the future time logic?

In my opinion, you should look at Heaven and Babylon Garden where in Heaven, time can be controlled (Since it's always bright and never dark there is the usual norm) while in Babylon, it's constantly flowing. Try to pull a 10000 year blink-of-the-eye time push on a rock and it becomes a fossil shortly afterwards. Not so much in heaven, especially where ages can last for millenia.