How long depends on many different factors.
One should be observant and never leave a girl unattended while she is bound.
If you are suspending her from her wrists I advise extreme caution, without the proper support it is very easy to dislocate her shoulders and or to suffocate her.
Improper application of bindings; from rope to handcuffs, can cause pressure ulcers, bruising and open wounds in as little as 20 minutes.
Never tie a knot against the skin or bury a knot were it is not clearly visible to the naked eye.
Be aware of the difference between reduced circulation and nerve compression.
With reduced blood flow there is first a sensation of tingeling and then pins and needles.
One may see thier slave wiggeling her hands or playing with her fingers and toes during this stage.
Shorty after the effected the area will go numb. It can take as long as 15 minutes.
Any time a limb is elevated above the plane of the heart the proccess is accelerated.
Reduced blood circulation is not ussually permamently damageing unless it goes to the point at which necrosis would set in.
I do not reccomend excceding 4 hour durations.
Sometimes if tied for an extended period of time a slave may become paralized in the effected limb for as long as hours or days afterwards; so I recomend re-positioning and or re-circulating blood flow every so often during a session if possible to avoid this.
If they cannot move their fingers or toes, at any time during the session, or are experiencing a sharp 'drilling" pain as opposed to a normal type of muscle tension spasm or the tingeling decribed above, you need to change the way they are tied becuase you have compressed a nerve and it can cause permanent damage in a very short period of time!
Do not let her lock her knees, especially if she is standing and do not put to much pressure on the chest with any bindings, or she can easily pass out.
Have a set of bandage scissors ready or a good knife in case you have to cut her down.
Untie her from feet to head so she doesn't fall over.
Avoid burning her by carefully removing the rope from where it is placed as opposed to tugging or pulling it off.
Don't expect a slave thats been released from a position they have held for 15 minutes or longer (tied or untied) to be able to jump up and run around. The joints often take more time to re-perfuse and movement should be done slow and safe.
[U]Be ready to help her.[/U]
Sub-drop is much more common after a heavy scene when a girl is bound. Allways be ready for aftercare and have plenty of ways for her to rehydrate.
Regardless of what else; if an emergency developes knowing how to treat for "shock" and being able and ready to preform CPR until EMT's can arrive is essential for any dominant weather you are using rope or not.
Allways plan ahead!!!