Yes, you can lock in for long periods such as 90 or even a 180 days. However you will take some pretty serious hits on your rate for locking in for such a long period. You also may have a problem with finding a broker that will do this for you since there are many issues that may come up before the closing date that will change the details of the loan. If a broker locks a rate and is unable to close by that date they will be punished by that lender for not following through by that said lock date. I would wait a little longer and consider locking for 90 days or less once you know for certain the final price and finish date. Rates should not change too drastically in the next few months in my opinion.

I agree with Brady. If you lock in now and then try to shop around for rates with a different lender it will tarnish your existing lenders reputation with their wholesale lender. When you lock a rate that lender is reserving a certain amount of money for you in anticipation of closing that loan. When you don't follow through with your commitment then that broker is held responsible for lock fallout. I would wait to lock until you have a more definitive answer on when your builder will complete your home. They tend to run into having to extend the deadline...