Constructing a new property

Dear members,
I am soon going to construct a 4 bed room ground plus one 250 sq.Yard (10 marla) house in bahria town karachi (btk). I wanted to know what is the approx construction time required for the same and which is the best month to start construction from?

Also kindly suggest is it advisable to sell my current place of residence and shift temporarily on rent so that i can manage the funds for the house con struction?