On average, the Yoyo (S) takes between three and four hours to charge fully.
When a device has a rechargeable battery in it, it is recommended to 'power-cycle' the device every now and again. A 'power-cycle' allows the internal battery to wear down fully before recharging it.
'Power-cycling' ensures that the battery receives a regular 'work out' to ensure that the lithium-ions are still functioning as they should be.
We would, therefore, recommend performing this as much as possible.
A moderate volume level will ensure you will receive the longest amount of playback from your battery.