I have the same thunder Power 15c 2100 lipo and I get 10-12 minutes hovering time on my rex - with 13tooth pinion - so you are getting correct times.

I have my voltage cut-out set to "high" as there is not really much between the settings and getting maybe most 1-2 extra minutes is just not worth damaging your expensive batteries!

A VERY IMPORTANT THING to know for newbies that I found out! The ESC low voltage cut-out protection is actually a percentage cut-out of your original batteries initial voltage.
Like I say - I have mine set on high - but what YOU SHOULD KNOW is NEVER disconnect your battery half flight and re-connect it! if you do this - DO NOT rely on your ESC low voltage cutout to save your battery... you need to mentally keep a track of how long you have been flying.

This is because, when you re-connect the battery, the ESC works out the low-voltage cutout based on the start-up voltage of the battery - and because the battery has now been used - that cutout voltage may - and probably will - be too low for your battery and might result in damage to your battery.

So if you have to disconnect and reconnect your battery half way through a charge - end your flight a little early or time your flights so you know when to stop!!!!

Note: I have a 25A ESC as well - the guy I bought my rex off had a neat idea - he csut some of the heat shrink away to expose the heatsink on the esc and mounted (with CA glue) a small computer fan to the esc with some small mounts.... works well and keeps the temp of the esc down makes a little noise but the heli is much louder than the fan anyway - lol