I've got plenty of 1100mAh batts that do that, the led on the fire button will stay lit, then turn off after so long through the charging cycle. Nothing to worry about.
It's the charger itself that will tell you when your battery is charged, not the battery led. If the led on the charger is red that means it's still charging, when it turns green, your battery is fully charged.
Vaping since April 2008 and still loving it :)
Stopped smoking April 2008, soooooooo easy :)