revised lesson

After watching my fellow peers model their engage phase of their classes last weekend I have altered my engage lesson previously taught last week, and tried it again with a different class. This time I focused more on developing the target language they will use later in the lesson while activating their schema. The last time I tried this class I did not introduce the language they will be using at this stage of the lesson.
I started the lesson off with a brainstorm on the board of different kinds of bad habits. Basically these were one or two word shout outs, which I let go at this stage of the lesson. I feel that getting the students involved at this stage rather than on full sentences was more important at the beginning. I then further progressed into the topic by showing pictures on the screen from the computer of different kinds of bad habits. I then asked questions such as "who has this habit?" "What is this habit". Again the responses were small and the dominant students answered. I asked 3 students and tried to personalize the feedback with them as much as possible by asking follow up questions, such as "how often do you eat hamburgers? and "what's your favorite hamburger?" ...
It was at this stage when I modeled the 3 sentences on the board that I wanted the students to focus on. They were:
1) I've picked up the habit of ___ing
2) I'm addicted to _____
3) I need to kick the habit of ____ing.
The students then did 3 minutes of pair work practicing these 3 sentences with each other before I nominated 3 students to say their examples to the class.  From this point we worked into the area of smoking and focused on that topic where students started to come up with reasons as to why certain groups of people start smoking while using the new modeled sentences.
The pair work worked really well at this stage of the lesson, even though it was short. The students all seemed to use this new language later on in the class also. My learners definitely feel more comfortable when speaking to other students as the conversations in pair work was great compared to when I utilized volunteering and nominating. I tend to have my students in one large horse shoe shape (U) for the start of the classes, so the pair work was just done with the person sitting next to them. In future lessons I need to think of a quick way to partner them up with other students, however I do not wish to spend too much time on this at the beginning of a lesson.
Overall my warm up, preview start to the lesson lasted 20 mins. Is this too long? Should there be a time limit on this phase before the study phase begins?