It wouldn't take them anywhere near that long. Voyager's original 70-year estimate was from the very farthest edge of the Delta Quadrant, 75,000 light-years away from the Federation. Borg territory extends much closer to the Federation, as close as 30,000 light-years, and Star Charts shows an offshoot section of Borg space even closer, a mere 5,000 or so light-years away (this might be the source of the Borg supercube from Resistance/Before Dishonor). And even the Borg's conventional warp drive is probably more powerful and efficient than Starfleet warp drive. So even without the subspace tunnels, they were probably within 15 or 20 years of invading the Federation.
Not according to "Mere Mortals" and an alarmed 7 of 9:
"She pulled him backward, off balance. Borg assimilation tubules extended from the steely implant still grafted to her left hand as she pressed her fingertips against his jugular. The tubules hovered above his skin but did not penetrate it—yet.
Around her and the admiral, the combat operations center became deathly quiet.
“If you do not escape beyond the Borg’s reach, you will never be safe,” she said, all but hissing the words into the trembling man’s ear. “They know where you are, and they are now committed to your annihilation. Even if you collapse the subspace tunnels, they can still reach you by normal warp travel. It may take them decades. Perhaps even a century.
But they will come. And when they do, your civilization will be eradicated. All that you have built, all that you have labored to preserve, will be erased from history. You cannot stop them, ever. As long as they exist, you will never be free.”
She's being generous in assuming that the Federation would have a century to demonstrate that even with
a huge amount of lead time, they'd still be doomed. There's no reason to think that the Federation actually has
7 of 9 was trying to alarm the admiralty, to shake them out of their overconfidence.
"If you destroy the tunnels, the borg will be here in a century"
more alarming that
"If you destroy the tunnels, the borg will be here in 15 years?"
Well - I guess we'll have to agree to disagree on this point.