Has to be signed off by two parties - leaving unit CO/OC and gaining unit OC/CO ; you're probably not going to be top of their list of things to do. Obviously it also takes time for the form to go from one unit to the next.
Then has to be sent to APC where it depends on how busy MS Reserves types are.
This will depend upon your current OC and future unit, equally your current OC is asked to comment on you as an individual, if you have played the system and been a bit of a lad then expect some payback which may influence your new unit, I completed such a form recently and took great pleasure in presenting an honest pen picture, suffice to say the process has stalled somewhat.
Just play the waiting game, it has to be confirmed by both units paper work and then up at Glasgow. Mine took about 2 months, it would of been shorter but my previous units clerk wasn't the quickest person to get things done.
I've switched cap badge three times now, as I move around the country for work and so transfer to new units. In my experience it takes many weeks or a few months, but most units will be happy for you to train with them 'on probation' or similar, as a newcomer, while the paperwork is sorted out. I expect informal phone calls will be made...
For what its worth, I transferred from another service to the TA and JPAC managed it in 3 days. There had been a little bit of correspondence between the 2 units beforehand but it all moved rather quickly!
Just had a quick read through of the new amendment to TA Regs 5.165 and inter-unit transfers are now the responsibility of the Bde/Div involved and MS Reserves will only get involved when they don't have the required JPA access (e.g. when moving between Divs), hopefully this should speed things up a little.