what time does shenzhen north station ticketing office opens

yes, the ticket office opens 24 hours, but you'd better go to collect your ticket in day time. or at least a few hours before the scheduled departure time.

but the last time I was there ticket counters has an opening/closing time but I failed to remember. I have this 7am schedule and I was thinking if 8 or 9 am the ticket counter opens, I won't be able to ride my train.

oh, i'm sorry, i forgot this railway station only operates high-speed rails, so it closed at night. i found this opening hours of 6:30—23:20, but i'm not whether it is correct or not. but one thing i'm pretty sure that the ticket windows must open earlier than the first train.
but is sitll suggest that you collect the ticket one day before you leave.