The frequencies of haplotypes may or may not be in linkage equilibrium. Linkage disequilibrium occurs whenever there are any departures from the expectations of linkage equilibrium. Linkage disequilibrium can be expressed numerically, as the difference between actual haplotype frequencies and those expected at equilibrium. The algebra is straightforward but tedious. D is linkage disequilibrium.
Recombination works to reestablish linkage equilibrium.
How fast depends on the rate of recombination. The explanation on p. 203 is quite clear and to the point. The human HLA genes are a multi-locus gene system. The human lymphocyte antigen system is amazing in several ways, most notably for its tremendous number of alleles at each of several linked loci, for combinatorial assembly of the peptide products those loci to produce an almost unlimited range of antibodies, and for somatic mutation to produce still greater antibody diversity. The HLA system also provides examples of linkage disequilibrium, which probably have significance for disease resistance.