You are doing all the right things to get the maximum life out of the core and the most numbers of holes.
Providing you keep the core cold (max warm) you should
get the proper life span.
DONT : Heat the core. Drill too fast. Apply too much pressure.
DO: Keep the core cool. Drill at setting 1 on a hand drill. Apply gentle pressure.
If the tiles are not fixed and you can drill them outside its easier to apply a continuous water supply via a hosepipe - over a drain etc.
If the tiles are fixed (and vertical) Then holding a wet sponge under the core will cool it down plus it will collect the dust at the drill site.