A deleterious allele (a) is codominant and starts at a frequency of 0.99.
The rate of decline depends on its fitness as measured by W.
A strong selection pressure where 90% of the heterozygotes (WAa = 0.90) and 80% of the aa homozygotes survive (Waa = 0.80), the frequency of allele a drops from 0.99 to less than 0.01 in 85 generations.
Under weak selection pressure where WAa = 0.998 and Waa = 0.996 it takes 1000 generations for the frequency of allele a to drop to 0.93.