OK, that might not seem simple. But, a star is simplistic in the sense that its entire future is quite predictable given its mass when this thermonuclear fusion process begins. We can predict how long this steady radiation will endure based on this mass. One might think that more massive stars have more fuel to burn, and will burn for longer as a result. Though more mass does mean more fuel, it also means higher temperature and pressure, which means a greater fusion rate. Massive stars actually live a far shorter life than less massive ones - lifetime is roughly proportional to M^(-2.5), where M is mass.
In the graph shown below, the approximate time a star spends in its main sequence is shown as a function of its mass ratio to the Sun.