while I'm on gateway emuNAND (with cartridge inserted), full charge in sleep mode last less than 48h on my normal 3DS.
I didn't check the exact time as I always notice it too late. But at around 30H of sleep mode, the battery LED is red.
I need to charge it every day to keep it alive.
I didn't test without EmuNAND/gateway. I usually use sleepmode to prevent relaunching gateway mode. When I'm not in gateway mode, I always use full shutdown.
I wish the 3DS would last as long as PSP sleep mode (more than a week)