Here's what it said about it:
Human Power to recharge the GBA SP Battery - Also Compatible with Nintendo DS
Turn 2.5 cycle per second to generate electricity
8 minutes of spinning provides 30 minutes of battery life (with light) to 45 minutes (without light)
Using the winding charger is not advised for more than 15 minutes (potential overheating problems)
Expect minor hand pain after 8 minutes of effort
More a gadget than a real lifesaver, for extreme situations or the pure fun of it