Cutler in his book Amalgam Illness around p 50 has a diagram of how people usually respond to the first year of chelation.
Around 4 or so months after Amalgam is removed and after a number of rounds of DMSA or DMPS the blood is largely cleared. Then mercury starts coming out of the organs into the blood. This is sometimes called the "dump" or "stall" phase. It can last until around 9 or so months but is varies somewhat from person to person.
Cutler strongly recommends that we chelate during the dump phase. If we do, as the Hg comes out, there is DMSA or DMPS in the blood to increase the odds that the Hg will be removed from the body rather than redistributed elsewhere--and cause symptoms.
Some people think that it might be better to just use DMSA or DMPS during the dump; however Andy likes us to use ALA so it will go into the organs and have a chance to escort some of the Hg out rather than have the Hg rip up cell membranes on the way out.
DMPS is well regarded in the Cutler groups for those who also have candida. DMSA tends to exacerbate a candida problem at doses over 25mg from my 2 yrs experience in the groups. Cutler provides many suggestions for dealing with it but when possible, folks tend to use DMPS. Many of the autistic kids choose to just use ALA. Their situation is somewhat different since they rarely had Amalgam fillings removed. Taking ALA almost immediately is common.
Remember you don't need large doses of chelators to get nice gains. I've been chelating for 1.75 yrs on an average of 16mg DMSA + 8mg ALA every few hours nearly every weekend. My bone density problem is almost gone according to DEXA and I'm off all medications. I have a ways to go but clearly I had an HM problem and chelation is working! : )