The road distance from Thief River Falls, Minnesota, to Glacier National Park (West Glacier entrance, near Kalispell, Montana) is approximately 750 miles (1,207 km).
Expect 11.5–12.5 hours of pure driving time, not including stops for fuel, food, rest, or border crossing (if entering Canada via US-2 alternative). The route traverses the Northern Plains and Rocky Mountain Front—scenic but remote; services are spaced far apart in eastern Montana.
Glacier National Park's Going-to-the-Sun Road (the iconic trans-mountain route) is typically fully open only late June to mid-October. In winter/spring, the park is accessible but the alpine section is closed. Check NPS.gov for current conditions.