Lifestyle and progress: How does it work?

Lifestyles are meant to take time to develop, because while traits reflect who a Sim is, Lifestyles reflect more how a Sim is living their life. Therefore, lifestyles are gained by doing activities or by being a certain way. Each has their own criteria and separate points counters. Since your lifestyle can change, they can also decay.

It will take a Sim about five full days to develop a lifestyle, assuming you reach the maximum points each day (around 20). When you've got 100 points, you get the lifestyle and will keep it unless you fail to do that thing for too long. Doing it enough once in 4 days should be each to maintain, as it decays at a rate of about 5 points per day. You will get a notification if your Sim is about to lose a lifestyle so you can take corrective action.
Other reasons they may go away exist. For instance, life circumstances may change as well, such as a Single and Lovin' It Sim finding love - the trait will definitely be gone after a time, if not instantly.