Switchover altitude depends on the high-altitude Mach number and the low-altitude IAS setting you program into the FMS. Those also vary with the Cost Index you set.
With a low CI, the switchover altitude may be very close to cruise altitude, because the FMS is going to revert to a low IAS (as low as 250 KIAS in the 744) very quickly. With a higher CI, the switchover altitude will be lower. If you manually program a low Mach number and higher IAS (e.g., .80/310), the switchover will be much lower. With a high CI (>250) and left to its own desires, the 744 FMS will program a low-altitude descent of >335KIAS, and will reach it closer to FL 200.
Also, without further direction, it is implied that you will retain your present cruise Mach in the descent until reaching the switchover IAS. Last I knew, the default low-altitude descent speed is 290-310KIAS above 10,000'. There is seldom a "do whatever you want" aspect to it.