mjames,
In very broad brush strokes "Closing" is made up of two key elements; (1) the final approval of the loan and the release of funds by the lender and (2) the recording of the deed at the court house.
In actuality, when the seller signs the paperwork has no real impact on the timing except in those cases (like VA loans for example), where the seller may have to sign some documents associated with the loan.
In your original contract was there a date selected for closing? In Oregon we establish a "no later than" date and, while I used to live in Ca, I don't recall any longer how it worked there. That date provides the target for the lender to prepare documents and get them to escrow so that the BUYER can sign and the transaction can close on time.
Typically, once the buyer signs the loan documents it can take 24-72 hours to get that paperwork back to the lender for final review/approval and release of funds. Once the money is released, the escrow company will record the deed, distribute the remaining funds as appropriate, and the deal is "closed."
Remember also that the closing date and the possession dates can differ too. Many buyers miss that point and have the moving truck scheduled on the same day as they sign papers...and sometimes that can be a very bad move (I know, I did it myself once before I got into this crazy real estate business).
Best of luck with your closing...if you have any other questions, the best person to ask is the escrow/title company.