From Genome Analysis Wiki
→How long does imputation take?
#r, # of rounds (-r or --rounds, --mle corresponds to 1-2 rounds)
Computational time increases linearly with m, n, r and quadratically with h. On our Xeon 3.0GHz machine, imputation with m=25K, n=250, h=120, and r=100 takes ~20 hours.
If you have a larger number of individuals to impute (e.g., > 1,000), we recommend a 2-step imputation manner http://genome.sph.umich.edu/wiki/MaCH_FAQ#Why_and_how_to_perform_a_2-step_imputation.3F.