
I just charge it when I feel it needs to be charged.
I don't usually charge it whilst playing it because I don't often stay on my 3DS long enough for it to need charging, especially since I just stick it on charge whenever I'm not using it (since it stops charging when full anyway).
If it does need charging whilst playing though, I'll just stick the charger in and carry on playing. I'm not stopping playing because it needs to be charged.