A lot of CO's will deny an requests for transfer when a individual has only arrived at unit straight from training.
Most will defer for several months/years until such time as they're sure that the reasons for transfer are the right ones. Possibly not just a reaction to not getting the posting you wanted. Trade training doesn't come cheap!
Or it could be within days if they sumise that you're a complete tosser and then the SSM/CSM/BSM will be there to pack your bags personally.
Change before starting Phase 1 or state a good case during.