When you know (she is madly in love with you) there is no bad time to tell her you want to spend the rest of your life with her.
You could serve her breakfast in bed one Sunday morning, propose during picnic at the beach, or whatever makes you feel comfortable. Hopefully if you're considering marriage you have an idea of what she would consider to be distasteful. Personally I think proposals are better when they are private. You don't need to be surrounded by thousands of fans at a football game or family members at a dinner table. Often times guys do this to put pressure on their (wife to be) to say "yes" in the moment.
A marriage should only take place when BOTH people want to get married!