Flew with a rejoiner recently, they said something about if you rejoin more than a year after your resign date, you start over with regards to staff number and grade/seniorority. Could be outdated though. They also said it took the company about a year from when she contacted them until they completed all the process and she flew to DXB the second time.
As you say it all depends on past record, company requirements and the reason you left in the first place (Almost all of the re joiners I have met had to leave for family reasons)