I'm getting married in Glacier national park & already applied for a permit, my guests are staying near the west entrance & we are exchanging vows near St. Mary entrance. My question is how long does it take from West entrance to St. Mary's entrance?

If by St Mary's entrance you mean St Mary's lodge, it is a approximately 25 minute trip plus time to get through the checkpoint.

yes♥

Depending on which road they are taking (Road to the Sun or highway 2) its easily an hour of driving just due to the quality of the road with the turns.