If it is an AB amp, probably not much; if it is a Class A amp or biased well into Class A it will draw the full amount of current or a substantial portion of it. This is easy to tell, just feel the heat sinks when it has been on but not playing music, if they are cool or lukewarm leave it on all the time. If you burn your finger don't leave it on. Most amps draw a minimal amount of current and sound better left on.
