Well...ideally it should last 2+ months. That's pretty situation dependent, though. Did you buy it in sheath, bud, or bloom? If the flower didn't completely develop under your growing conditions, that's likely to be the reason it isn't lasting. The subsequent blooms will be acclimated to your conditions and bloom for longer periods
I was thinking around 6 weeks or so but they could last longer. Temperature can often affect how long a bloom stays out, the warmer it is the shorter the blooming period. Not sure how that impacts on your spicerianum though.