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

Charging it completely before first use is the recommended way. https://support.google.com/nexus/answer/6102350?hl=en

None, just do it. If you allow your phone battery to go nearly empty before recharging it, then it will last a little longer, as you avoid the Memory Effect. This problem however becomes less with each new battery technology (NiCd, NiMH, Li-ion, Li-poly, etc.)

There is no need to charge the phone when you first get it with new phones today. The comment to charge your phone all the way as soon as you get it is a misconception carried over from older batteries that were used.

People have misconception with a new phone should be charged to 100 percent so that the battery is calibrated but in reality it should not matter what you do it or not. This notion was true earlier when they were batteries needed to be primed i.e. they have to charge 24 hours before using them but due to the advancement in the battery technology now our batteries do not need to be primed. So you can start using your phone normally as soon as you get it. Some  people says that keep your battery between 20 percent to 80 percent but this can have a memory effect on the batteries (though it is highly debated whether it has or not) but one thing is for sure that do not drain you battery completely and try to charge when ypur battery is about 10 percent that way you will decrease the charging cycle and will prolong the life of the battery. From my personal experience I can say that charge whatever way you like because the batteries have became so much better it doesn't matter gow you charge or discharge them.

Most phones are launched with a half full battery. You can start using your phone out of the box. With modern batteries you do not need to charge them for many hour before first use.

For 20 minutes Max. Initially the phone needs just a quick start and a rapid leap to boot, and for that only that much battery is required which is sufficient for normal running just to give the phone that initial 'push' ie 40-50%.

It is highly recommended that you charge it completely to 100percent and discharge it to about 10 percent ONLY ONCE in a month. The other days, keeping your battery between 30 and 80 percent should be good for it :-)

Charge it as much as u want.  No damage is gonna occur if you start using it straight away. So if you wana check random features of your phone as soon as you take it out of box you don't need to wait for it to get full  charged first. Also no need to charge further  once it reaches 100 percent.  No need to do 8 hours of charge initially. Nothing gonna happen. Charge your phone as much as possible rather than going for full charge and discharge. Hope it helps.

If u buy a new phone dont charge it right away drain the battery to 0% by using the phone  then after the battery gets drained  switch off & charge it for 4 hours this will increase your battery life in the longer run