How long is too long for a variable name?

I suppose, I'm looking for both answers that are both technical and opinionated in nature.

How long is too long for a variable name?

I always try to make my variables as short as they can be while maintaining the proper level of meaning. However, as some "sub-routines" increase in complexity, I'm finding the need to create longer variable names to continue to have them be meaningful. Does the length of a variable have an impact on performance?

For instance, I just used MBReadPerSecondAverageAfterLastFlushToLog, which is as close to supercalifragilisticexpialidocious as I hope I ever come.

Thanks,

Matt

p.s. Sorry if you now have a Mary Poppins song in your head.