Attack rate does not eat up aura life. The only thing that matters is how many creatures are in the aura.
auras decay overtime even with 0 creeps walking through them..
maybe when u upgrade them the rate of decay increases without creeps walking through them... someone test it out ... i dont get home until 8pm
When you upgrade a tower/trap/aura all the corresponding Tower Stats (Tower Health/Attack/Radius/Attack Rate) are upgraded by an exponent of 2.
So 1st upgrade will be 2^1 or +2 to tower stats.
2nd upgrade: 2^2 or +4 to tower stats.
3rd upgrade: 2^3 or +8 to tower stats.
4th upgrade: 2^4 or +16 to tower stats.
5th upgrade: 2^5 or +32 to tower stats.
Most people with Huntresses/Monks put many more points into Tower Health and Tower Attack than the other 2 stats. So let's build an example character.
Tower Health: 200
Tower Attack: 300
Tower Radius: 100
Tower Attack Speed: 80
If you upgrade an aura with 5 upgrades, here are the example stats for that particular aura:
Tower Health: 200 + 32 = 232
Tower Attack: 300 + 32 = 332
Tower Radius: 100 + 32 = 132
Tower Attack Speed: 80 + 32 = 112
As we all know, adding stats into a character (wether it be Hero Stats/Trap Stats) is also exponential, ie. there's more of a difference between 100 - 120 than there is 300 - 320.
So according to this example we're "improving" Tower Attack Speed "faster" than the other Tower stats. So proportionally Traps/Auras will fall faster because you're adjusting the Tower Attack Speed higher than any other stats.
This is my speculation BTW. Sorry to pass off the info as fact, as it clearly isn't backed by any member of TE.
And I'm fairly sure an Electric Aura attacking every .3 seconds will fall faster than one attacking every .6 seconds, assuming the Tower Health is the same.