If it's done right it will be around ten seconds, as stated in the wiki.
1. The loop has to close in on the neck. Correct noose (slipknot, "simple noose", whateverthefuck), not too much friction.
2. The arms must be tied/cuffed behind the back if the anchor point is within reach (to avoid pulling oneself up).
3. The chair/ladder/etc must be kicked away as to not be able to stand back on, not having other shit that can be used for this purpose as well.
4. The rope must hold.
5. The anchor knot must be correctly chosen and tied correctly. Bowline/snuggle hitch/another slipknot/etc.
6. The suicidal must be undisturbed for 30 min or more.
7. The rope must cut off both carotids completely (not too thick).
8. The anchor point must hold.
9. The feet might have to be tied.
10. Lubrication might have to be used.
11. Alcohol might have to be consumed to supress SI.