The shut down could have been done within 30 minutes at maximum after the first shooting.
But shouldn’t have been, given that there were only 2 people shot and it appeared to be just an interpersonal conflict involving the shooter, his target, and one person who tried to intervene. Locking down a 2600 acre campus with 25,000 students in that scenario wouldn’t be reasonable — but they absolutely should have sent out an alert with basic info on what had happened, and the fact that the shooter was still on the loose, and a PHYSICAL DESCRIPTION OF THE SHOOTER (which apparently wasn’t even included with the message that finally went out 2 hours late).
You don't shut down a community of 26,000 because someone got shot. At the time it looked like a single incident. Do they close your town down when someone gets shot? If they had shut down the campus where would they have put the thousands of students who were on campus for class that didn't live on campus? Have you ever been on a campus as large as Virginia Tech? What you're proposing would have been logistically impossible. And if it had been done and students were congregated somewhere in the open and the shooter started picking them off there'd be people blaming the police for that too.