There are three opinions among the Rishonim regarding the overall length of the tzitzit. According to the Rosh, four finger width is enough. Rashi says six and Rabbenu Tam says 12. The Tur, Smag, Darkei Moshe and Prisha all poskin according to Rabbenu Tam, and that's the opinion brought down in the Shulchan Aruch.

These 12 fingerwidths are measured not from the hole, but from the start of the first knot at the edge of the tallit (or tallit katan). Since tying the double knots shortens the overall length and because from the hole to the first knot isn't counted, you need to start off with strings considerably longer. Usually strings are at least 100 cm. After you insert them in the hole, you get 50 cm on each side. By the time you get them tied, you'll probably have 35-40 cm, although various factors can change that.

According to the Grach, 12 fingerwidths is 24 cm, while the Chazon Ish holds that it's 29 cm. So even if you want to follow the more stringent opinion, and even if some of your strings come out short, nonetheless they're probably at least 30 cm long.

The Beit Yosef entertains the possibility that you measure from the hole. The idea is that tzitzit are comprised of the tied section ("gid") and the loose section ("anaf"); since the part from the hole to the first knot lays close to the garment, it's more akin the the gid than to the anaf. If we were to integrate this opinion, it would afford us a considerable leniency to be used in a bind. 

Most tzitzit are longer. The Mishna Berura notes that the Ri made his longer than that so that if a bit of the ends got cut off he'd still have enough length. But the Aruch Hashulchan states quite clearly that there's no mitzvah to make your tzitzit longer than the required length.

Another point to factor in is that the Gemara says that the loose part should be twice the length of the tied part. In a previous post I explained that despite this, people typically leave the untied part more than twice the length of the tied section.

The Rambam writes that regardless of the length of the strings, the tied part should come out to one-third of the overall length, but adds that with all-white strings there are opinions that say we need not adhere to this ratio. The Beis Yosef speculates that the reasoning for the latter opinion may be that the Gemara says:

נוי תכלת שליש גדיל ושני שליש ענף

And the former opinion holds that the word "techelet" is employed here simply because that's the standard type of tzitzit.

The Chazon Ish writes that based on a careful reading of the Rambam, it's okay for the loose part to extend beyond two-thirds of the length of the tied part (O.C. 3,10).