Isabelle90,
First I am not a rope Master but the question is reasonable. It depends on what you wish to do with it. For rope corsets you need at least 25 feet, to do a complete body tie I would say 100 to 200 feet. I would say that shorter pieces come in handy for tieing a sub/slave from cuffs to bedposts, etc.

Oh safety tip, with nylon rope if you do get the cinch down metal clamps to make the loop with you still don't use it against flesh.