I think they need to be exposed to lots of examples throughout all levels, including at Ele.  But I tend to focus on them more explicitly around the end of Pre-Int as well.

One warning, though, it is very complicated and the 'one tense back' rule is a very loose one that can lead to lots of overgeneralisation.  I'd recommend focusing more on highlighting the time of the statement, the time of the (intended) action, and the time of the report and focusing on how these change what tense is used.  And make sure you cover the changes in people, place, etc, too.

I guess I'd recommend something like the following regarding what to deal with first, second, etc:

1. Introduce Present Simple statements that are reported a couple of days later, focusing on people/place changes with no change in time.
   (eg.  Tom, while at work: I drive here.  =>  Susan reporting Tom, while outside work:  Tom said he drives to work.  Focus: I changes to Tom/he; here changes to work).
2. Introduce recent Past Simple statements that are reported a couple of days later, focusing on the time change.  (eg.  Tom, on Monday: I finished the report. => Susan reporting Tom, the next day:  Tom told me he had finished the report.  Focus: finished changes to had finished now that you are talking about a past action (report finished) that happened before another past action (talked to Tom).  Rather than highlighting 'one tense back', highlighting Past Perfect for an action before another past action will be very logical).
3. Introduce various statements in which the time follows one of the above two rules, but which contrast the people/place more.
4. Introduce various statements in which the time varies depending on how the time of the action relates time of the report:
   (eg. Tom on Saturday: I'm going to do it on Monday.  => Susan reporting Tom on Sunday: He said he's going to do it tomorrow. => Susan reporting Tom on Monday: He said he's doing to do it today.  => Susan reporting Tom on Tuesday:  He said he would do it yesterday.  [or 'was going to do it'].  Focus: As long as the action is still seen as in the future, the tense doesn't need to change; but once the action is seen as in the past, the tense changes.)

I find this approach feels more logical - not everyone's the same, though.  See what works best for your students and what you feel most comfortable with.  And a good international series of course books will have a decent syllabus built in - get a series to refer to and it can help a lot.

Sorry, I meant the bullet points 1-4 would each be taught at different stages.  Perhaps step 1 towards the end of Pre-Int, step 2 early in Int, step 3 mid/late Int, step 4 late Int or Upper-Int.  It can be flexible though.