Three days is the sweet spot for many first-time visitors: enough time for the historic core, Mount Royal, major food stops and at least a few neighbourhoods without racing.
One day
A single day can work for a stopover, but you need priorities. Old Montreal, a Mount Royal viewpoint and one evening neighbourhood form a reasonable first-time structure.
Two days
Two days gives you one classic sightseeing day and one neighbourhood-focused day. You can add the Plateau, Mile End, Jean-Talon Market or the Lachine Canal without making every hour feel scheduled.
Three days
Three days is where Montreal starts to feel coherent rather than like a list of attractions. You can divide the trip into historic Montreal, central landmarks and food/neighbourhood exploration, with room for weather changes.
Four days or more
Extra days are useful if you like museums, cafés, markets, nightlife, cycling or slower neighbourhood exploration. Montreal rewards repetition and wandering more than many attraction-heavy destinations.
The key variable is pace
Travellers who want to check off landmarks can compress the city. Travellers who want to understand it should leave time for the spaces between landmarks: commercial streets, parks, cafés, markets and residential neighbourhoods.
MontrealCanadian.com editorial guide. Practical details can change; verify hours, admissions and seasonal operations with official operators before visiting.