It would be a good idea to chelate after removing the fillings, especially if you are having health problems.
How long you should wait depends on the chelator you plan to use. I think only ALA has a 3 month waiting time. Other chelators can be started soon after. I believe the recommendation is to wait a few days to a week before you begin chelating. This gives your body a chance to clear out some of the mercury released during filling replacement before chelating releases more.
The waiting time on ALA is a little more complicated to explain and has to do with concern that taking it too soon after replacement may carry loose mercury into a person's brain.