How long does it take to create an excess slot on the UMR from HRC's point of view?

Depends.
Are we talking about creating a position by changing the MTOE? That is a review that is done annually at the GO level. You'd be waiting a while.
Are we talking about simply putting you in a position where you are qualified, but excess? That is a simple matter of assigning you to a double-slot, and is done routinely in most units.
Are we talking about actually assigning you as excess in a 9993 (or other 999X) position? Also, easily done at the company or battalion level.
*EDIT*
Seeing your other question, I surmise that we are talking about authorized AGR positions, which is a bit more complicated. Positions coded for AGR are set by HRC on the MTOE, but authorized at a lower level (usually MACOM) for actual assignment of personnel. You may be encountering this issue as one company had the correct position authorized but another did not, causing you to be assigned to the authorized position.
A request of this type will need to be dual tracked.
One will need to go through the CoC for endorsement up to the MACOM.
Another will need to go through the AGR management liaison for your command to HRC for the change in PARA/LIN authorization.
Your career manager also need to be in the loop so they can get PCS orders generated.