Coin cells are good for a few hundred mAh?http://www.dipmicro.com/store/BAT-CR2032
225 for example.
Can the attiny be put in sleep, to wake up when PIR pulls a pin low?
Then send out some pattern - see the virtual wire library for the kind of sequence to send that a receiver can pick up on and interpert correctly.
Best method is to measure the current during the actual conditions, sleeping and transmitting.
Probablys see ~5-10mA while sleeping depending on the PIR, and a jump to 15-20nA while transmitting.
So 225/10 = 22 hrs.
Better off with a 1000mAh LiPo, can find them in niice small flat package.