It's hard to answer your question since you don't provide an in-context citation, but I suspect that "before record closed" refers to the evidentiary record that is before the administrative judge.

what is meant in these clearance decsions rendered by the Defense Office of Hearings and Appeals when it says "before record closed".  does this mean before sf-86 was filled out or before it started being processed, or what..

I don't have the answers to your question, but to research it yourself, you might refer to Security Clearances and National Security Information: Law and Procedures written by Sheldon I. Cohen for the Defense Personnel Security Research Center. This book may be downloaded as a 17.5 mb PDF file here:

http://antipolygraph.org/documents/security-clearances.pdf

You might also review some of the recent industrial security clearance decsions rendered by the Defense Office of Hearings and Appeals:

http://www.defenselink.mil/dodgc/doha/industrial/

Is there anything that can be done to show mitigation of future use when taking into account the recentness of use?

You could falsely deny your past drug use and use countermeasures to pass your polygraph, but I do not advocate this course of action. I think persons seeking security clearances have an ethical obligation to answer the relevant questions truthfully.

Past use of marijuana (even frequent use) does not necessarily preclude one from ever holding a security clearance, but the recentness of your marijuana use may well pose a problem. However, if you lie about your marijuana use and it comes to light in the course of a background investigation, it will be very difficult for you to ever hold a security clearance.

I am considering interviewing for a job that requires a TS clearance (paperwork to be filed when you start the job), and a lifestyle poly.  I have used marijuana frequently in the past and within the last 2 years.   

I assume I will be asked about drug use during a lifestyle poly and being caught lying on the SF-86 will be grounds for denial of clearance.  Is there any way I would be able to deny past drug use, or past use within the last 2 years (i have a passed drug test from about 1.5 yrs ago) on the SF-86 and the poly.   

basically should I even attempt to secure this job with my recent use of marijuana or should I wait a few years to allow enough time for mitigation of the use.