How long a culture will live on slant is strain dependent. My yeast bank was so large at one point that I routinely pushed things out to two years or more if I did not care if I lost the strain. I spend so much on a yeast cultures today that I am unwilling to push the edge of the envelope. One can afford to push things with cultures that cost $6.00. However, one cannot afford to do so with cultures that cost at least $100.00 (I have three cultures in my bank that pushed $200.00 each by the time shipping and import duties were added to the equation).
With that said, I no longer collect and maintain yeast cultures that can be obtained via the home brew trade. My time is more valuable than the cost of a commercial yeast culture. In my humble opinion, the cultures that one should collect are those that are difficult or impossible to obtain. For the home brewer who is unwilling to lay out the big bucks for culture collection cultures, strains from the vault and platinum/private collection strains are worth banking. Brewery strains are also worth banking, and so are strains native to one’s location. Unless one lives in the absolute sticks where everything has to be ordered, banking standard production liquid cultures will get old quickly.