Like with any battery, how long a charge lasts depends on how you use the device. Under typical active use, you can expect a full day of speaking with your group.
If you use Volume Boost mode, you can expect less time because of the higher power used. That's why we recommend turning off volume boost when you no longer need it.
Milo integrates a high-quality lithium polymer battery. The number of charge cycles before the battery capacity is substantially reduced is very high, so you should expect multiple years of very active use before the battery capacity is meaningfully degraded.