An Objective can be created in the My Objective section. After answering a few questions, the game will recommend target exercise times and frequency, such as exercising 60 minutes a day, three times a week for four weeks, for a target time 720 minutes in total.
After creating an objective, you must use the exercises marked with blue flags to earn progress toward it. Once you've met your total target time, the achievement will unlock.
You do not actually need to exercise as frequently as you designate, you simply need to reach your target total time. You could do one long session to complete the objective in one day if you'd like.
To make it easy on yourself, change the frequency to the lowest possible settings: once a week, 30 minutes a day, four weeks, for a total time of 120 minutes.