It isn't the time that grain is crushed, but the temperature and humidity that it is stored in. If milled malt could be stored at 60F in a 4% humidity environment, it wouldn't go slack any faster than unmilled malt.
But in the real world, we all have differing humidity's and storage conditions. Cracked malt wants to absorb moisture from the air and that degrades the enzymes. For moderate humidity of 30 to 40%, it will only take about 4 or 5 days for the enzymes to noticeably degrade. By that, I mean anywhere from 5% to 15% less efficiency.
This isn't the end of the world, though. You may be surprised at how many very popular breweries have to purchase milled malt and do so in one to three month volumes. Sometimes, those breweries take steps to modify the humidity of their silo.
I like Phlm63's analogy, though. It is kinda like coffee. You can never go wrong with milling your grain the day you brew.