The critical factor with Li-Ion is, as slim-slag says, total time since manufacture. Don't buy a new one till your old one is on the way home, and then ask for the date of manufacture before you pay over your hard-earned.
I don't think Li-Ion care much about number of cycles - mine does 45 minutes in the laptop, and always has, despite the manufacturer's quoted "up to 2 hours". It shows 2 hours to go at switch-on, counts down logically to about 1 hour 15, then beeps its emergency power-down and shuts off.
I have a mains power supply and a car power supply (also works in the aeroplane) - belt and braces, but serves me well.