Question

How long should I charge a new phone before the first use?

Answer 1

No need to charge it for 12 hours. Your new phone will probably get charged within 2-3 hours. The best you can do is to make the battery go through 3-4 full charge-discharge cycle. By that i mean charge the battery to 100% and use it until the charge has dropped to 2-3%.

Answer 2

Considering that the majority of available phone batteries are either Li-Ion or Li-PO (3.6 or 3.7 nominal voltage, 4.1 or 4.2 fully charged), and neither type suffers from memory effect, there is no need to perform any special acrobatics.

That said, lithium rechargeables have a recommended storage voltage level (about 3.84 volts or somewhere around 40-70% full), at this charge level they age much slower.  This is why batteries in new products are typically shipped at about 60% charge and not at 100%.  A fully charged or fully discharged battery will age more quickly (fully discharged is the worst long-term condition).