Is the property a short sale?
No matter this should not be a factor as to when attorney review begins because once the home owner not their bank and the buyer signs the contract and both get a copy, they have 3 days to begin attorney review according to Opinion 26 notice to buyers which should have been the cover page notice BEFORE your contract which your agent had you sign. If they did not have you sign it, they just broke the law.
After contracts have been signed you will have 3 days to have your attorney send out the first review letter which usually disapproves of the contract in it's current format however with a few proposed changes they will find it acceptable and binding then the other attorney has 3 days to respond and this can go back and forth for a while or both attorneys could agree to hold on attorney review from going any further until a certain act has taken place.
Sorry no cut and dry number, it depends on your situation.
If you need a good attorney I could recommend a few I've worked with in the past who know their business but my disclosure, always perform your own due diligence. Past work and success is not always an indicator or guarantee of future performance.
Attorney Review benefits the seller, because they can still market and show the property.
Buyers should push to get it out as quickly as possible.
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