Really those small phals will have shorter spikes so staking I don't think is really necessary. Should you choose to do it, though, you can start as soon as the spike is long enough to be able to be bent and tied to the stake. Maybe 3ish inches long? Play with it.

I'm sorry but I have to disagree. I don't want to see you break off that wonderful spike. I let my phals. grow them naturally and only spike them once they start falling over into other plants.

Bob what are you disagreeing with? Read my first sentence