You should have left it in overnight (for the very first charge). Too late to accomplish that now, but it will still help to leave it plugged in all night - ASAP!
The capacity and long-term performance of these batteries is affected by achieving a really, really high state of charge on their first one or two charge cycles. (BTW, I'll SWAG that it's Silver-Oxide, in a semi-liquid solution, but that doesn't matter -- most others are similar. NiCAD is the exception, you should let those batteries run all the way down between charge cycles, because of the "memory" effect which they suffer from partial charge cycles.)
It is IMPOSSIBLE to over-charge a Dexcom battery. When you leave the charger plugged in for a really long time, a small control circuit (which is integrated on the outside of the battery) stops presenting inbound charging voltage differential to the battery, when it has detected that the battery isn't absorbing any energy.
A last comment: When using most kinds of batteries (Li-ion, NiMH, etc.), it is bad to run them down too far. If you want it to last much more than a year, then DON'T wait for it to start blinking with just one mar left. If possible, recharge whenever it has reached 2 bars. You're putting in less % of maximum-energy-strorage-capacity per charge, but the maxiumum number of charge cycles which the battery will accept goes up by a much higher factor.