Your lock will only expire if you don't get a lock for the appropriate amount of time. You can elect to lock for the entire duration of construction or stay floating until later down the road. Ultimately it's your responsibility to tell the lender when to lock you (you and your lender will discuss how that process is done and how you'll stay on top of the information you need to make an informed decision. Keep in mind, when you do a long term lock like this, lenders will ALWAYS require you to pay up to (possibly more) 1% of your loan amount as a deposit so you don't hop to the next lender everytime the market improves by 0.125%. Like stocks, you can't un-sell them once you pul the trigger. That being said, most lenders now offer a one-time float down option incase rates improve, then you can take advantage of the better rates.
At the end of the day, find a competent loan officer who can explain your options to you and set your expectations properly.