As for the duration of a round with ALA+DMSA: the ALA round is as described above [*]. You can overlap it with your DMSA round in the following ways:
[A] start DMSA (every 3 hours), when feeling good after a few days add ALA (same schedule), FINISH ALA AND CONTINUE DMSA until symptoms subside or for at least two more days
[B] start both DMSA and ALA every three hours, quit ALA as per ALA schedule [*] and continue DMSA for up to 15 days to mop up what mercury was dropped when quitting the ALA round and clean up the heavy metals from the blood.
[C] do A or B above with intermittent rounds of DMSA alone.
Should you get into trouble while taking ALA alone (e.g. when you stop taking DMSA for some reason and discover that you cannot safely continue without it), my experience suggests that it is OK to add DMSA in the middle of an ALA round at the last safe established dose of DMSA and while adhering to a schedule of every 3 hrs (every 4hrs at night). Once you add DMSA to an ALA round, you have to continue taking both these chelators until you can safely finish the round of ALA, and CONTINUE TAKING DMSA for up to 15 days to stretch the round a few days beyond the end of your ALA round.
Any decision should be informed by your SYMPTOMS. Listen to your body.