In order to make a siyum on a mesechta, does one have to leave over the last piece for the siyum, & if not, how soon after completing the masechta does one have to make the siyum?
It is customary to leave over the last piece of the Mesechta to learn at the siyum, in order to allow the attendees at the seudas mitzvah to participate in the conclusion of the learning. One should do so.
If one finished learning the Mesechta, he can make a seudas mitzvah anytime that day. If it is impractical to do so (such as on a fast day), one may make the seudas mitzva the following night. [Under some circumstances, it may be possible to make a seudas mitzvah on the following day; consult a Rav.]
In general, it is permitted to learn the bulk of a Mesechta and then wait until a later date to arrange a siyum. One may even do so to avoid fasting on Taanis Bechoros. However, most poskim suggest that one may not do so in order to eat meat/drink wine during the nine days - one may only make a siyum for that purpose if he actually finishes the Mesechta in a normal course of learning during the nine days.