How far is the Niagara fall from Montreal, Canada if traveling by car?

It's a little over 400 miles (about 640 kilometers) and probably 6 1/2 to 7 hours.

over 400 miles not much farther

Should take you about 5.5 hours to get there. High speed, no traffic.

Roughly 7 hours. You might try travelling through the United States in an effort to avoid the heavy traffic of Toronto.

5.5 hours to Toronto, then a little over an hour to Niagara. Definitely avoid Toronto or the highway past Toronto on a weekend afternoon or during weekday rush-hour.

When you get to Toronto, take highway 427 south, then the Queen Elizabeth Way (QEW) highway west, then follow the QEW over the Burlington Skyway to Niagara Falls.

Give it 10 hours. 7 1/2 to drive. You sure want to take some breaks as well.