How long do I have to use my grains once they have been crushed?
The sooner the better, obviously, but I have waited up to a couple of months without any major impact.
I wouldn’t wait that long or longer if given a choice. You should be okay out to 6 or 8 weeks.
Paul
I would try to use them within a month. You could use them after two, but I would start to worry about staleness. If you keep them in the freezer, I would say up to six months. Otherwise I would make some bread with them, or give them to the birds and critters. 
Here’s a sample data sheet from Briess:
They say six months. I always use mine within an hour or so.
I had a batch milled and then had a heart attack. It was 3 months before I used them and they were fine.
How many pounds did you grind by hand for that to happen? Just kidding! I suffered the same fate without grinding grain…and damn glad to be able to enjoy the hobby without limitations from it.
Hope you are too.