How long should Earnest Money tie up contract?

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I bought a vacant lot that is zoned for commercial development. The lot is still vacant, and just had an offer from an interested party. The contract price is for $200,000.

However, the buyer is wanting to put down $1,000 for earnest money, and is requiring 180 days for a feasibility study. If the feasibility study comes back not satisfactory then the buyer can walk and be refunded his money. The buyer also has an option for another 30 days for a non refundable $1,000.

I do not want to tie up the property for half the year, and the deal fall through due to a feasibility study while not receiving anything.

Any tips or what is typical in this situation?