There is no maximum flow-rate that can be vented to the atmosphere, but the hazard analysis should consider the potential risk of dispersion, radiation, and overpressure as part of the vent system design. Flare systems are often used at large hydrogen production facilities as one means to prevent a large unignited cloud from forming but will depend on the specific application.

The routing will be dependent on the system design, size of release, and evaluation of the hazards.
Smaller systems are rarely vented to a flare due to complexity, availability and permitting considerations.
Facilities handling large amounts of hydrogen such as production plants will often have a flare system
since they have more capability for this additional onsite infrastructure.