I want to answer your question because I thought it would be a little interesting to tell you some insights based on experience. I hope this helps you find some answers and motivates you to do something better in the future.

• There shouldn't be a fixed time frame and I think it will vary a lot between users. It all depends on what you do in this forum because reputation and trust depend on yourself and every user will get it in different timeframe.
• Quality posts, constructive and useful for other users who want to read them. For your progress, then quality posting is one of the factors to determine that you have the desire to have something of value in the forum. This will help you grow into a useful user for other users and you only need to hear advice that really helps you build your morale for the better.
• It will really depend on your progress in posting, understanding something right and doing activity on the forum. You have to get an activity number between 775 - 1030 as a condition, but you also have to get the first 1000 merit to make it happen. Your consistency in making valuable posts will greatly affect how much merit you get, so the activity and quality of your posts will determine how long it takes to rank up to Legendary.