alright, first off: i was joking. no really, half my posts in this topic aren’t serious, mainly because i myself can’t take doomsday seriously — i’m not saying i don’t believe in doomsday, i just can’t take it seriously.
that being said, and now in all seriousness, you might be right: four to seven minutes can be too much. they can also be way too little: a black hole can take years to consume a planet the size of Earth. here’s a quick explanation of what a black hole is: it’s a point (as in “a zero-size body”) called singularity with a preposterous quantity mass. if you were to do it old school and calculate its density as mass over volume, you’d find out its mass is infinite, which under old school gravitation laws would mean singularities are quickly swallowing the universe.
lucky for us that theory has been proved wrong already. what happens with singularities is, their mass will create a (really strong) gravitational force. things will be attracted to the singularity. and there’s some point, which varies greatly depending on the mass of the singularity, at which there’s no return. there’s this distance from singularities which we call “event horizons”, explaining exactly what they mean is quite dull, but it works for now to ask you to think that whatever gets closer to the singularity than that distance has no coming back.
that weird system made of a bodiless point, the singularity, and its influence zone within the event horizon, is what we call a black hole. right. so what is the size of a baby black hole? honestly, i’ve no freaking idea. but here’s an interesting figure: the minimum mass for a primordial black hole (i.e. one originated at the Big Bang) seems to be 1 000 000 000 000 kg (= 2 200 000 000 000 pounds). it would take 10 000 000 000 000 000 000 000 000 000 years for one such baby black hole to swalow the whole Earth. (which i suspect is longer than the Earth will live, anyway, and at any rate long enough for you not to care).
for more information: great article on Universe Today, and then some more.