Mas tipid ba sa kuryente pag longer hours nakabukas ang AC?

We’re using a split-type inverter AC (Hisense 2HP) for a 33sqm unit.

Before, we would run it for about 6 hours at night and our bill averaged around ₱4K. Recently, we reduced usage to just 4 hours, but the bill is still around ₱4K, if not a little higher.

What’s the ideal setting for this kind of unit? Should we actually be running it for longer hours instead?