In a life-threatening type of situation, my guess would be as soon as possible. Usually if a now order is important (at least where I work) the resident or doctor will verbally tell the nurse that the order is in. For me, if a now order is put in and no one tells me, when I see it, it'll be done. I usually cringe if a now order has been sitting for a couple of hours without being done, but it also depends on what the order is for. Some things don't have to be neccessarily done immediately, just sometime during the shift.
Call me crazy, but I feel that if a doctor thinks an order is important, the nurse should be included in the plan of care.
Although that doesn't always happen, I think the good ones will keep you informed.
Hope this helps!