Good question. According to a professor at Michigan State University---whose name I don't recall--anytime you include more than 10 percent rye in a mix with blue...the rapid growth of the rye will cause it to dominate the stand. You will have mainly a perennial ryegrass lawn. But at least it will happen quickly. Ideal conditions mean something like 80 degree soil temp, uniformly moist, well-prepared soil, and starter fertilizer. Perhaps seed lightly buried--about a quarter inch.
Opinions vary--experienced people may chime in.
For instance Pickseed's perennial rye, Cutter II, lists a normal 24 days to first cutting, in their "Technical Product Specifications". Blazer 4...only 21 days...(it is a hydroseeder's favorite).
Of course, if you are laying sod--the green grass is instant. Make agreement to take the customer's check when the job is half complete. In any event...your agreement should state the grass is not his until it is paid for...and you reserve the right to pick it up and lay it elsewhere.