Here are numbers that are helpful when using alfalfa and grass height as an indicator of NDF content.  In general:

Begin cutting 100% grass stands should be cut when nearby alfalfa is 14 inches tall to achieve the desired 50% NDF. 
Begin cutting 50/50 mixed alfalfa and grass stands when nearby alfalfa is 22 inches tall for the desired 44% NDF.
Begin cutting 100% alfalfa stands when alfalfa is 28 inches tall for the desired 40% NDF. To achieve the desired NDF it is necessary to begin cutting at NDF values slightly lower than the desired value assuming some harvest and storage losses which the above alfalfa height values reflect. For example, 28-inch-tall alfalfa is closer to 38% NDF but you will need to start then to end up at 40% NDF.

Predicted days to cut are based on daily NDF increases for grasses of 1% point, 50/50 mixed stands of .8% points and alfalfa of .5% point. Typically, NDF increases about 0.8 to 1.2/day for grasses expecting the lower end of that range in cooler weather and the higher end in warm. Alfalfa NDF increases about 0.4 to 0.7/day again figuring the lower end of that range with cool temperatures and the higher in warm. Predictions are adjusted for the coming week's weather and we are assuming normal temperatures and growth.  Seems optimistic but we will trust the forecast for a few 70°F days in the next week!

In the tables in the pdf, locations around the region are listed where we have measured alfalfa height.  You can use the location and elevation as a guide to conditions that may be similar to your own.  Always check your fields to verify your heights and conditions!