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Some lenders i've talked to will lock the rate in as far as 6 months out...I'd suggest shopping around.
It is very rare to find lenders to lock in a rate for free for more than 60 or 90 days. Most lenders won't even let you pay to lock in the rate. This costs thousands of dollars over the life of the loan in the long run when you lock in a lower interest rate when they can stick you with one slightly higher.