How long will the LabQuest battery pack last?

The long-term life of the battery will vary, but you can expect 500 full charge/discharge cycles for LabQuest 2 (300-400 full cycles for original LabQuest) before the battery will need to be replaced. A charge from half-way to full would count as half of a charge cycle. It is fine to charge the battery every night, even if the battery is only partially discharged. There is never a need to condition the battery by regular full discharge/charge cycles.

The battery takes 10-12 hours to completely charge. It is safe to leave the battery charging indefinitely. The time that a battery will last after being fully charged depends on the sensors used, but in most cases you can expect batteries to last for a full school day, or 6 hours, between charges.

When using LabQuest with a computer, the LabQuest cannot operate on USB power alone. It must be charged or the LabQuest must be connected to AC power.

Exposure to temperatures over 35°C (95°F) will significantly reduce battery life. For example, leaving a LabQuest in a hot car all day will reduce the battery life. This is the most common reason for premature battery failure.

As a battery reaches the end of its useful life, the run time will become shorter and shorter. Eventually the run time will be too short to be useful, and you will want to replace the battery. When the battery reaches the end of its life, you can calibrate the battery so it will give a more accurate reading of its remaining charge.
     See TIL 2075: How do I test the battery on the original LabQuest?

LabQuest batteries are warranted for one year. In typical school use, most batteries will last for about three years. Replacement batteries are available from Vernier:
LabQuest 2 Battery, order code LQ2-BAT, $19
LabQuest Replacement Battery, order code LQ-BAT, $19

See also:
TIL 1780: Optimizing LabQuest battery life
TIL 2741: LabQuest 2 Battery Troubleshooting and FAQs
TIL 2077: Original LabQuest Battery Troubleshooting and FAQs