Long life sensor - realizeable?
I don't mean to put a dampener on your project but your question is a bit to broad in it's current state. There are many factors that will affect how long the battery will last. In addition to the more obvious ones like broadcast duration and transmit power you also have to consider things like:

Data rate, higher data rate radios tend to need more power due to the extra processing required (and depending on the RF modulation scheme, a more linear, i.e. less efficient radio) But they can sometimes use less energy overall as they need less time to send the same amount of data, your mileage may vary (4G is more energy efficient than 3G despite being less power efficient as it does more work in less time)

Power draw of the sensor and main MCU, when designing long life sensors, the idle power draw of the system becomes really important (as does the active power draw of the MCU and the sensor), you need to get the active and idle current draw as low as possible. A 1mA average current draw will drain your battery in about 40 days. This average current has to also take into account the self discharge rate of the battery (which can be much higher than the datasheet at elevated temperatures, most figures are taken at 25'C, sometimes they give you figures at higher temperatures but not always)

The efficiency of the radio, a more efficient radio will allow you to transmit bigger data packets (or transmit them at a lower data rate which can increase your range as it lowers the receiver noise floor) or use less power to send the same amount of data with the same noise floor.

Receiver sensitivity, more sensitive receivers will pick up weaker signals meaning you can usually transmit at a lower power setting assuming no strong interfering signals on the same channel.

Another important factor is your antenna, if you've got a big enough antenna you can detect a garage door opener on mars. There was a team a few years ago who managed a wifi link over some 270km Line-of-Sight using ordinary USB wifi dongles because they had 3m parabolic dishes at either end (just try doing that with the stock antennas those things come with!).

As for the transmitter power well, it will depend partly on how much interference is present on your transmit channel, Cubesats have been able to talk back to earth from orbit with 0.1W radios, so you may be able to get away with something on the order of a few milliwatts transmit power (0dBm is another way of saying 1mW in Rf parlance, 3dBm is 2mW, 10dBm is 10mW, 20dBm is 100mW, mW = 10^(dBm/10), just so you know as many RF chips are specced in dBm rather than milliwatts). 

A good place to start is probably with a zigbee system as some of them are designed with low power operation in mind and many have provisions for a nice big whip antenna. If you've got access to a decent multi-meter and you transmit for long enough for the readings to stabilise (most DMMs have update rates of only a few Hz so they don't register fast changes well). Measure the idle current and then have it transmit for a while and measure the transmit current (the actual data content has little to no effect on the power), then you can just take (idle current * idle time  +  transmit current * transmit time)/total time and that shout give you some number in mA/hr. Multiply that number by the mAh rating of your battery and you'll get roughly the number of hours the battery will last.

when in doubt, try it out