Transistor Junction voltage
It depends on the transistor. Look in its datasheet for a specification called "Vcbo", which stands for "maximum Voltage, collector-to-base, emitter open".

If you exceed this value, the transistor may or may not be damaged, depending on how much current you allow to flow &mdash; i.e., how much power the transistor disspates while the current is flowing. Generally speaking, it is the temperature rise from this heating that produces permanent damage.

Sometimes, a transistor is deliberately operated in reverse-breakdown mode, although the more heavily-doped base-emiiter junction is usually used. For example, a transistor operating in its Zener breakdown region makes a pretty good wideband noise source for some applications.