I think it depends on what you're seeking. For example, some people use Seroquel or Zyprexa as PRNs because those meds calm aggression and impulsivity very quickly, and that's due to the sedative properties of those meds. But Seroquel and Zyprexa, much like other APs, take quite a bit longer than a few hours actually kick in and handle symptoms. People are relying on the sedation and calming effects more than the actual anti-psychosis/anti-manic bit.

The other thing is... it's easier for a "PRN" to work faster if it's already built up in your system. For example, let's say you're taking... idk... risperdal everyday. Well, if you already have risperdal built up in your system because it's your regular med, then an increased dose will work faster compared to how risperdal would work if you had never taken it before. Or so this is what my pdoc tells me. That's why I think it's common for "as needed" APs to be the same AP you're already taking. So if you're taking abilify (I think?), then maybe 10mg is your normal dose and you have a 5mg PRN dose. That's usually what pdocs do, and it is recommended in the literature. I can find literature links if you're interested in learning more.