The only sources I found on this come courtesy of chaburas.org and say one can wait a bit but not a lot.
Maharam Mintz claims that when one reaches the end of a tractate, he
should delay learning the end until an appropriate time comes for
making a siyum. Minchat Yitzchak (2:93) qualifies this by
distinguishing between waiting a little and waiting a lot. One who
finishes a tractate the week before Pesach may draw out his learning a
bit so as to make a siyum on Erev Pesach, but one who finishes in
December should not save the last page for the four months until he
needs it to avoid having to fast. He says that in such a case, one's
siyum would not be made out of simcha, but rather out of a desire to
satisfy one's physical needs.
Regarding a siyum during the nine days specifically see here
However, those who complete a significant portion of learning (e.g. a
tractate of Talmud) during the nine days solely for the purpose to eat
meat are not universally praised. In fact, the Aruch HaShulchan, while
allowing such behavior (suggesting that at least in this way, people
will learn Torah) does limit the people participating in this meal to
just Torah scholars associated with this learning project [Aruch
HaShulchan Orech Chaim 551:28].
Nevertheless, the Mishna Berura, among
others does allow broader participation in the meal, extending it to
all those who would normally be invited to such a celebration [Mishna
Berura 551:75]. Once the week of Tisha B'Av begins (from the Sunday
before Tisha B'Av), the number of participants at such a siyum should
be limited to a minyan aside from the relatives of the person making
the siyum and those who are helping with them meal [Orach Chaim
551:10; Mishna Berura, ibid. note 77].
