I'm in TX & I've used option periods on the contracts. Bottom line is you can put whatever period you & the Seller agree to. For motivated Sellers, I've found that they don't really care what you put down. My contracts are usually for 30 days & my option period for 2 weeks.
If you are talking about a lease option period I typically use a 30 day, and yes when it expires unless I cancel it then I am obligated to follow through on the contract.