I normally charge my N900 overnight using older Nokia charger ACP-12E(800mA output) via CA-146 adapter. Charging takes ~ 2.5 hours. I tried to use AC-3E(350mA) with the idea that charging will take longer and thus the battery will start draining later so I will get few more hours autonomy, but with that charger charging almost never ends. Phone stays connected to it like 8 hours with green LED light not appearing. And the same charger was filling up my E61(1500mAh battery) for 4.5 hours. CA-146 gets warm when charging N900 with AC-3E.

PS. I have noticed that charging with ACP-12E instead of default charger that came with N900 tops up battery closer to 100% before green light shows and charging stops.