There does not seem to be an established guideline for how long a shower curtain should be. Both long and short shower curtains have their advantages, but the most important factor is personal preference.
Tanya Lacourse of Apartmenttherapy.com suggests that both long and short shower curtains have merit. For example, long shower curtains prevent water from spraying out more readily and will hide marks and stains on the tub from onlookers. On the other hand, a short shower curtain is less likely to develop mold and dries much faster. Other than that, there are no inherent advantages to either a short or long shower curtain.