Los Angeles to Montreal: total time 68h 45m, 3278 miles, in other words 3 hours 15 minutes short of 3 full days.

Now to give you a little more information that you might want to know:

Average speed is 47.68 mph, including all waits between trains.
If we remove the time between trains at Chicago and Schenectady, then you have 63h 49m on trains between scheduled on/off points for an average of 51.37 mph.

If we break this down train by train, you have:
Southwest Chief: 2256 miles, 42h 35m, average 52.98 mph
Lake Shore Limited: 800 miles, 14h 07m, average 56.67 mph
Adirondack: 222 miles, 7h 07m, average 31.19 mph.

If we further break this down by major segments of the trip, you have:
Southwest Chief (all on BN&SF)
--Los Angeles to Albuquerque: 924 miles, 16h 40m, average 55.26 mph
--Albuquerque to Kansas City: 895 miles, 17h 05m, average 52.39 mph
--Kansas City to Chicago: 437 miles, 7h 50m, average 55.79 mph
Lakeshore Limited
--Chicago to Cleveland (on NS): 341 miles, 6h 11m, average 55.15 mph
--Cleveland to Schenectady (on CSX): 459 miles, 7h 47m, average 58.97 mph
Adirondack (on CP in US, not sure on Canadian side):
-- Schenectady to Rouses Point: 173 miles, 4h 12m, average 41.19 mph
--Rouses Point to Montreal: 49 miles, 2h 55m, average 16.80 mph

The times for the segments will not quite add up to the totals because I left out the station dwell time at Albuquerque, Kansas City, etc. Albuquerque to Kansas City is as slow as it is because of Raton Pass and mountainous territory in northern New Mexico. Across Colorado and Kansa it is very fast. Rouses Point to Montreal is so slow because in includes the customs inspection, and of course terminal slack.

Please excuse me if I am explaining the obvious in the following paragraphs:

Terminal slack, or padding, is time added to the schedule between the last few stops or only the last stop before the end point to give some make up time so the train can be late but still get into its end point on time. In the case of the Adirondack this appears to be about 35 minutes. For the Southwest Chief into Chicago it appears to be about 50 minutes. The arrival time at Albuquerque also appears to have some padding added.

All the information used to develop the numbers above was pulled off the Amtrak schedules. If you are new to this, these can all be found at amtrak.com, then go to trains & destinations, then go to the section of the country, in this case Midwest and northeast, the go to the train name, then go down and find timetable. This will give you a pdf file with the train schedule that you can print or download or both.