Does the US fly its flag at half mast on Government buildings when there is a tragedy, or someone famous dies?

Yes. But nor sure for how long. Doesn’t Canada do the same ?

It originally was lowered for government leaders (president, senator, supreme court justice, etc.)

Over the years it has been cheapened and used far too frequently for non-politicians and non-government people.

Nowadays it’s lowered for almost every mass shooting. (Not saying that mass shootings aren’t tragic, but putting the flag at half mast is a departure from historical precedent.)

The United States flag flies at half-staff (or half-mast) when the nation or a state is in mourning. The president, through a presidential proclamation, a state governor, or the mayor of the District of Columbia can order flags to fly at half-staff. Most often, this is done to mark the death of a government official, military member, or first responder; in honor of Memorial Day or other national day of remembrance; or following a national tragedy.. https://www.usa.gov/flag

There are circumstances when individuals post a US flag at half mast when not a govt. proclamation. These are often overlooked by officials today.

Mast is usually used for ships or at Navy bases, on buildings it would be staff. Minor technicality, but basically means the same thing.

No. It depends on who died and what the president orders.

I get that,but how long does it go on for?

“How long should an US flag be flown at half mast?
The U.S. President can dictate by proclamation when, and for how long, the American flag is flown at half-mast. According to the Department of Veterans Affairs, the flag should fly at half-staff for 30 days following the death of a current or former president, and for 10 days after the death of the vice president and other government officials.”

Additionally, he/she can determine other occasions for it to be flown at half-staff and for how long: President Biden issued the proclamation on Friday, April 16, after eight people were killed when a gunman opened fire inside the Indianapolis FedEx building. Following the mass shooting, President Biden said flags would fly at half-staff through to sunset on Tuesday, April 20.

I think it became a constant thing in the US after 9/11. Since then, I sometimes think I never see a fire station or city wall with a flag above half-mast.