That is a great question and one that has not been asked on this forum before.
We recommend to charge new sealed lead acid batteries for 12 hours before their first initial use. This initial 12 hour charge will give them a deep charge which will help condition them for a long lifespan. During the initial charge the battery charger's light might indicate that the batteries are fully charged after a short amount of time, however leave the charger on the batteries for around 12 hours to provide them with a deep initial charge.
After the initial 12 hour charge then recharge the batteries after every ride, even very short rides, to keep them fully charged whenever the scooter is not being used. A deeply discharged battery pack should recharge in around 5-6 hours with most battery chargers, however there is no harm in charging the batteries overnight as the charger will go into float mode so it does not overcharge the batteries after they are fully charged.
Short rides will require considerably less recharge time depending on how long the scooter was used for.
Electric scooter controllers have a built in low Voltage protection circuit that will shut the motor off when the battery pack reaches an 80% discharge level, so you do not have to worry about letting the battery pack go below 20% of its charge because the controller will not allow that to happen.
Sealed lead acid batteries do not need to be 80% discharged for the first ride, they could be discharged any amount on the first ride as long as they were fully recharged afterwards. Their capacity will increase during the first 20-50 deep discharge and recharge cycles as the batteries are being formatted, so the deeper they are discharged during these first uses the faster they will be formatted to provide their peak capacity and ride time.
SLA batteries do not develop a memory like Nicad batteries do so they can be used for any amount of time from a few seconds up to they are 80% discharged without effecting their performance.
Here is a sealed lead acid battery lifecycle chart to provide a better understanding of how these type of batteries perform during their lifetime.
Please let us know if you have any questions.