There are a few factors that dictate when this happens. Such as, water temp, age, and health of the female, all play a role in when the eggs are deposited. Essentially, the 'saddle' stays until the female molts, and immediately after doing so, she mates. The eggs will then pass through her body, and be fertilized.
I *think* that adult dwarf shrimp take ~3 weeks to go from saddled to visible eggs, under her tail. It takes ~5 weeks more for the eggs to hatch. Again, this is all conditional, based on water temp (higher temperatures increase the metabolic rates of the shrimp, and decrease maturation and hatching times). Unhealthy shrimp (underfed, or stressed), may drop their eggs, or take longer to mature. Oh, I don't mean to give the impression that warmer temps are a good thing. Along with an increased metabolism comes a shortened life span.