Hi,
I receive my Onyx Boox A60 today. How long at least should I charge it initially (first charge)? Does it matter?

For sure. Charge it for 12-14 hours.

When I first got mine, I charged it until the blue light dimmed (it never goes out completely) which indicated the battery charge cycle is complete. That's how I've been recharging it ever since.

Plugged into an AC wall socket with the AC-USB adapter, it usually takes about 3 hours.