I've decided to test this out. I just charged my DS Lite, and while on the charger I have loaded up Metroid: Zero Mission and put it in "Easy Sleep". Now I will unplug it and check periodically to see how long the battery lasts.
From what I know about GBA sleep mode, it only turns off the screen and speaker, while the cartridge and system circuitry remained powered.
Nintendo states that the DS Lite battery will last from 5-19 hours, depending on gaming conditions and which level of brightness settings are used, but sleep mode is not mentioned. So, maybe after this I'll check NDS game sleep mode so I can compare it with GBA. Mario 64 DS should give a good baseline.
The DS Lite is brand new. Some Canadian big store retailer sold it on Ebay for $79, not knowing how much it was really worth. I have probably only played about 5 hours on it thus far. I will post the results when finished.