How many generations does it take for a population to reach Hardy-Weinberg equilibrium?

The Hardy-Weinberg equilibrium is not possible in natural populations. It depends on the following never happening:

1. Genetic drift, which is common in small populations and occurs after population bottlenecks (look at the cheetah, they have very little genetic diversity)

2. Nonrandom mating - how many instances of mating do you know of that are completely random? It does happen, but not too often. Inbreeding can occur in small populations (like snow geese) as well.

3. Migration

4. Natural selection

5. Random mutations - This is probably the least likely to happen; they are rare and often detrimental to the organism, but still serves to disrupt the H-D equilibrium.

If any one of these 5 things happens in a population, then H-D equilibrium fails, so it's more of a theoretical situation than something that is possible.