Timeline for Counter offer

[John Thurmond]

I am in Florida and I just received an offer on a property I am selling (offer A).  I countered a few hours later the same day.  About 2-3 hours after that, I received another offer (offer B).  Offer B is higher.  

In section 3 (Time For Acceptance of Offer and Counter-Offers; Effective Date:) of offer A, there is no date given for the contract to be signed by all parties, but Offer B has a 24 hr stipulation for the response.  By law, am I required to give the Buyer for A two days to respond to my counter, or am I allowed to withdraw the counter?