Had this tablet for one day, its showing 50% .. battery, when should I recharge..

You can go ahead and charge it to 100%. Maintain it by topping it off periodically. Try to avoid a complete discharge, but if this does occur, simply recharge at your earliest convenience. Each drain reduces the maximum capacity of the battery a little bit.

There are many variables that effect battery life, aside from the battery itself, it can seem quite overwhelming. Here is a table with some good information. The last column, Lithium-ion, is the battery type commonly used in cell phones and tablets. Please consult the documentation that came with your device to confirm this is indeed the case.
Frequently
asked question Lead acid
(Sealed, flooded) Nickel-based
(NiCd and NiMH) Lithium-ion
(Li-ion, polymer) How should I prepare a new battery? Battery comes fully charged. Apply topping charge Charge 14-16h. Priming may be needed Apply a topping before use. No priming needed Can I damage
a battery with incorrect use? Yes, do not store partially charged, keep fully charged Battery is robust and the performance will improve with use Keep some charge. Low charge can turn off protection circuit Do I need to apply a full charge? Yes, partial charge causes sulfation, Partial charge is fine Partial charge better than a full charge Can I disrupt a charge cycle? Yes, partial charge causes no harm Interruptions can cause heat buildup Partial charge
causes no harm Should I use up
all battery energy before charging? No, deep discharge wears battery down. Charge more often Apply scheduled discharges only to prevent memory Deep discharge wears the battery down Do I have to worry about "memory"? No, there is no memory Discharge NiCd every 1-3 months No memory How do I calibrate a "smart" battery? Not applicable Apply discharge/charge when the fuel gauge gets inaccurate. Repeat every 1-3 months Can I charge with the device on? Some UPS systems simultaneous charge and deliver current. It's best to turn the device off during charge; parasitic load can alter full-charge detection and overcharge battery or cause mini-cycles Must I remove the battery when full? Depends on charger; needs correct float V Remove after a few days in charger Not necessary; charger turns off How do I store
my battery? Keep cells above 2.10V, charge
every 6 months Store in cool place;
a total discharge causes no harm Store in cool place partially charged, do not fully drain Is the battery allowed to heat up during charge? Battery may get lukewarm towards the end of charge Battery gets warm but must cool down on ready Battery may get lukewarm towards the end of charge How do I charge when cold? Slow charge (0.1): 0-45°C (32-113°F)
Fast charge (0.5-1C): 5-45°C (41-113°F) Do not charge
below freezing Can I charge at hot temperatures? Above 25°C, lower threshold by 3mV/°C Battery will not fully charge when hot Do not charge
above 50°C (122°F) What should I know about chargers? Charger should float at 2.25-2.30V/cell when ready Battery should not get too hot; should include temp sensor Battery must stay cool; no trickle charge when ready Table 1: Best charging methods. Strenuous demands cannot always be prevented.

http://batteryuniversity.com/

http://www.techrepublic.com/blog/tablets-in-the-enterprise/how-and-when-to-charge-your-tablet-battery/

There's no harm in keeping it on charge all the time. Unlike older rechargeables, newer Li-ion batteries don't need to be broken in by running them down and then recharging.