I seem to be having starting the altezza these days.

(I don't drive it much, maybe 10-15 times since may)

This is the third time since August that the battery is totally drained. (I try and start it at least twice a week and run it for about 30 mins)

It just seems a bit weird that the battery is drained in a little over a week of inactivity.

Is there something that could be draining the power or would that the battery just need replacing?

How old is your battery? Could just be time for a new one hopefully.
Does you car have an aftermarket alarm fitted?

Didn't start again this morning either. Ran it for over a half hour yesterday.

Battery is about a year old.
I have an alarm/immobilizer from Meridian.

How is the fluid level in the battery?

If the battery is dead flat then your cars alternator isn't powerful enough to charge it fully. You'll need to put it on a decent trickle charger, something making 18amps + for 12-24hrs.
Also check the fluid levels as Stephen suggested. You could get yourself a cheap hydrometer from repco or supercheap to see if you have a faulty cell, that's only if you don't have a maintenance free battery though.
Only other thing I can suggest is take the battery back to where you got it & get them to check it out for you. At least then you can rule out if the battery is the problem or not.