Check the guage on the slate river at crested butte. If it is over 300cfs, OBJ is probably runnable. Good flow is usually 600-900cfs on the guage. In general, the higher the better.
Crested Butte is tough to hit at good flow sometimes. It will run big for a week, then we'll plan a trip. As soon as we get there it gets cold and snow and the water level drops. I've been at the OBJ campground several times in June when it snowed. In general Late may to mid june is the time to be there. This year, I bet it will be memorial day weekend.