Mutation Rate in Multicellular Eukaryotes
The real mutation rate of any organism depends by too many factors both external, like the environment, and internal, like the error rate of the DNA polymerases and DNA-repair enzymes and even from different region in the same genome (the mutation rate of non-coding DNA tend to be higher than the one of actively used DNA).

In general however it is possible to determine an average mutation rate for a given organism and usually it is expressed in mutation per base pair (or single nucleotide) per generation.
If you want to know roughly the mutation rate per gene or per genome you just need to multiply the mutation rate by the number of bases of the gene or genome.

Good papers on the topic are:


  Drake JW, Charlesworth B, Charlesworth D, Crow JF (April 1998). "Rates
  of spontaneous mutation". Genetics 148 (4): 1667–86.
  
  Ossowski S, Schneeberger K, Lucas-Lledó JI, et al. (January 2010).
  "The rate and molecular spectrum of spontaneous mutations in
  Arabidopsis thaliana". Science 327 (5961): 92–4.
  
  Nachman MW, Crowell SL (September 2000). "Estimate of the mutation
  rate per nucleotide in humans". Genetics 156 (1): 297–304.


If you want to know about some specific organism you have to dig into the literature...