With lithium chemistry batteries as are used in modern portable devices, there are two rules:
Always fully charge before first use. It will tell you how long to charge it for in the manual. That's the minimum length of time, there is no harm in keeping the device on charge overnight for longer as all lithium battery powered devices have charge circuitry which prevents the battery being overcharged no matter how long it is on charge for. You only need to do that once before first using it.
Second, never fully discharge the battery. Most devices will prevent you from fully discharging it as they'll say the battery is dead and shut off before it really is empty, but to be safe, once it is below an indicated 20%, you should really be considering recharging.
Unlike nickel chemistry batteries which some people say worked best with a full discharge and recharge each cycle (there is increasing evidence that either wasn't true or certainly isn't true of recent nickel cells), it is generally agreed that lithium batteries work best by only ever being partially discharged and then topped up.
In other words, after the first initial full charge, put your tablet on charge again whenever it is convenient.