First of all crack is nothing but a slightly modified cocaine formula. It has been modified to make the most addictive kind of cocaine that has ever existed. That being said, crack might give you a different high than cocaine but its behavior once in your system is pretty much the same, therefore it stays in your body for relatively the same time, namely:
In your blood or saliva - from 12 to 48 hours so no worries here.
In your urine - from 1 to 5 days (in a few very extreme cases, crack might be detected up to 3 weeks later in urine) - Drinking huge amounts of water is a popular measure to deal with this but if you're still not sure the day 4, ask a friend for urine that's cleared of any illegal substance. Judging by the widely spread drug presence in our society you better have friends who are 13 years old or younger (and are up for doing this without it being completely inappropriate).
In your hair - The worst case scenario as crack can be tracked in hair even years after the last contact with the drug. Bleaching or dying your hair is rumored to work, but it doesn't. There's always the option of shaving every hair on your body and claim that you're a professional swimmer or that your religious beliefs do not allow you to keep any hair. If they have a go at you because of this you can sue them and get one year's worth of salary at once in a chunky compensation after winning the case (which you will).
Please bear in mind that all these timings depend on if the usage took place in small amounts for the sake of trying something new or if it was one of those 4 day weekends where anything goes and by the end of it you are thinking "I don't even like my job so what's the worst that could happen if I just don't show up this week?".