Once the blooms fall off and you no longer see any new buds, it is time to cut the spike off about one inch above where it grows out of the plant. IF you orchid is growing a new leaf, it is time. Let the orchid put its effort into the new leaf and not the old spike.
Having said all of that, there are people who would say cut the spike just above the second or third node from the bottom and your orchid will branch and rebloom on the same spike. Though this is usually true, a young orchid will expend lots of energy into reblooming and will weaken itself in the process by not developing more roots and leaves. Plus, unless the orchid is in very good health, the second blooming on a phal orchid spike will create much smaller blooms than the original. I always recommend cutting spikes off after the blooms are finished and allowing the phal to grow healthy leaves and roots and prepare for the next round of blooms.
Good luck to you on whatever you decide to do.