You'll lose 5% in 24 hours......
Actually, a new quality (and most not so quality ones as well) Li-Ion cell should charge up to 4.20 Volts (100%) and stay there for days.
There is a commonly believed myth that Li-Ion cells have a "rested" voltage that is lower than "Hot off the charger". This is not precisely true. The only reason a Li-Ion cell will have a lower voltage, say, 1 hour after it comes off the charger, is cell degradation. Once a Li-Ion cell drops to 4.00 Volt after being charged to 4.20 Volts, within 24 hrs, it's time to recycle it. At any rate, if your Li-Ion voltage drops 5% in the first 24 hrs, it's time to recycle.
As to how long Li-Ions will keep their charge, it depends on the cell manufacturer and quality, but generally a Li-Ion should keep 80% of it's charge for 6-12 months. Some protection circuits, have a parasitic drain which causes more rapid self discharge of the cell, but they should still come close to 6-12 months.
Dave
Edit: After reading my post again, it sounded like I was saying Jarl was incorrect. That is not the case. He is quite right. I just wanted point out that 24hrs after charging, a Li-Ion can have lost anywhere from 0-20% capacity.