If have read conflicting reports on the half-life of mercury in the brain. I would like to hear some estimates from this community. I am also wondering how long chelation therapy would take to remove mercury from the brain.
Since you're concerned about mercury from fillings, you may have already taken a look at the wikipedia article about dental amalgam toxicity. It would be worth reading through some of the sources listed, too.
I assume you're an adult, I don't know if you're male or female. Children are more sensitive to mercury than adults because their brains are actively developing. If you're female, you may become pregnant at some point and there would be some concern about fetal mercury exposure.
However, according to a 2003 WHO report, the amount of mercury ingested from dental amalgam is low, with exposures estimated from 1ug per day to 12.5ug per day, but the vast majority of people are exposed to less than 5ug per day. The mercury from fillings is elemental mercury, which is poorly absorbed, less than 0.01% is taken up, with smaller amounts reaching the brain.
Removing your existing fillings probably isn't worth it, as the removal process would expose you to more mercury than just letting the filling sit there.
I don't know if this will make you feel better or worse, but coal fired power plants are probably a larger source of mercury exposure than your fillings. The mercury from the coal plants is methylmercury, which is much more efficiently absorbed than elemental mercury. This bioaccumulates in the environment, so that fish have relatively high levels of mercury in them.
As far as your perceived concentration problems, we really can't say if it's caused by the mercury or not. If it concerns you, you should see a doctor. And it may be more efficient to treat the symptoms through ADD medication than trying to remove mercury.
I have read reports of amalgam fillings producing as much as 50 µg per day of vapor with a 80% uptake through the lungs. This is what worries me. There is evidence that the half-life of mercury in the brain is many years.
The study below (2014) suggest the half life in the brain (where it counts for mercury poisoning), may be much longer than thought: on the order of years or decades.
The retention time of inorganic mercury in the brain--a systematic review of the evidence. (https://www.ncbi.nlm.nih.gov/pubmed/24368178)
Also: article about this study (http://www.speciation.net/News/A-new--study-finds-Inorganic-mercury-stays-in-the-brain-for-years-if-not-decades-;~/2013/12/29/7042.html).