The main factor limiting FM signal reach is often the height of transmitter and receiver antennas over ground, not the power. The Earth is curved and antennas must be in (or very close to) the zone of direct visibility (you should be able to see one antenna from the top of another).
Longer waves can bend over the Earth curvature, and short waves (the source talks about 3MHz and somewhat above) can reflect from the ionosphere to increase the distance. However 103.3 MHz you talk about is too high to benefit significantly from any of these two.
More can be found here.
Assuming that Earth curvature is not a problem, the distance depends on both the power of transmitter and sensitivity of the receiver (and sensitivity of the receiving part largely depends on antenna construction and quality). Hence it is also difficult to say without knowing these details.
Finally, if there is any material like building walls between transmitter and receiver, the limiting factor most likely will be the thickness and properties of this material (wood? bricks? iron?), not the distance.