Here's some basic guidelines.
Cash transactions can be closed in as little as a week if both parties are so motivated. 2-3 is more common as it allows for inspections, etc.
For most transactions with mortgages figure 5-6 weeks assuming the buyer is pre-approved prior to making their offer, and if their not pre-approved then their generally wasting their time making an offer as any smart seller will simply say, "Thanks but no thanks, come back when you're pre-approved" The only definite exception to this is if the buyer is using a USDA loan which can easily take 10-12 weeks or even longer as first the local loan officer has to put everything together (3-5 weeks) and then submit it to the USDA which is running 4-6 weeks out and then assuming they have no issues that need to be resolved it comes back to the initiating lender and they finish up (another 3-7 days)
In general unless you're planning on using a USDA loan or buying with cash, figure 6 weeks as the norm.