Have you verified that CH_PD stays low when the ATTiny is asleep? I'd consider putting a 10k pulldown on the CH_PD line so it defaults into powerdown mode. Do you have a multimeter? Insert it between VCC and U1 and between VCC and US1 to verify your sleep currents [1].
The 3.3v regulator could certainly be improved. Once you've verified that your sleep currents are within the expected range (<1mA), consider swapping it for something more efficient. A board like [2] should work reasonably well if you set UVLO to the minimum value of 0.3V [3]. This should get you to a few months of battery life.
A large capacitor (500+uF) on VBATT will stretch this a bit further. You want something with low ESR (the batteries should be around 400 milliOhms total; try to stay under 100 milliOhms). Anything here [4] would be pretty reasonable (supercaps are also a good option).
If you want to go even further, switch to an ATtiny85V and run it directly from the batteries. (This will work with your ATTiny85 as well, but you'll drop below its operating voltage while there's still a fair bit of usable charge left in the batteries.) Instead of wiring to CH_PD, you can wire it to the enable pin on your regulator (unfortunately not exposed on [2]; it's pin 6 on the TPS61200). You'll need to level shift the ATTiny<->ESP8266+sensor signals. The advantage here is that the ATTiny power doesn't take an efficiency hit from a converter and the ESP8266 and sensors can be 100% powered down when not in use. This is what I'd do if creating this as an original design on a single board.
The 3.3v regulator could certainly be improved. Once you've verified that your sleep currents are within the expected range (<1mA), consider swapping it for something more efficient. A board like [2] should work reasonably well if you set UVLO to the minimum value of 0.3V [3]. This should get you to a few months of battery life.
A large capacitor (500+uF) on VBATT will stretch this a bit further. You want something with low ESR (the batteries should be around 400 milliOhms total; try to stay under 100 milliOhms). Anything here [4] would be pretty reasonable (supercaps are also a good option).
If you want to go even further, switch to an ATtiny85V and run it directly from the batteries. (This will work with your ATTiny85 as well, but you'll drop below its operating voltage while there's still a fair bit of usable charge left in the batteries.) Instead of wiring to CH_PD, you can wire it to the enable pin on your regulator (unfortunately not exposed on [2]; it's pin 6 on the TPS61200). You'll need to level shift the ATTiny<->ESP8266+sensor signals. The advantage here is that the ATTiny power doesn't take an efficiency hit from a converter and the ESP8266 and sensors can be 100% powered down when not in use. This is what I'd do if creating this as an original design on a single board.
[1]: https://openhomeautomation.net/esp8266-battery/ [2]: https://www.sparkfun.com/products/10255 [3]: https://www.circuitsathome.com/dc-dc/tps61200-board-modifica... [4]: http://www.mouser.com/Passive-Components/Capacitors/Aluminum...