I believe there are really two elements to this question... one is what the IRS will allow as a tax free gift, and two.. what the lender will allow for the home purchase. The IRS will allow $13,000 per annum to be given from one person to another on a tax free basis. So, if parents hypothetically were giving to their son and daughter-in-law, each parent could separately gift to the son/daughter-in-law. Each would ultimately receive $26,000, or a total of $52,000 tax free to the couple. All tax advice should be verified by your CPA or other tax professional.
Each lender has different guidelines for parental gifts, when they can be given, how much can be applied to the down payment, etc.