Cramping and red blood due to first trimester miscarriage should not last too long. Unfortunately, it is hard to say just exactly how long is too long, considering the circumstances and possible danger associated with miscarriage.
If you suspect you're having or had a miscarriage, without a doubt the most important thing to do next is go to your OBGYN or regular physician for tests.
If the bleeding is far more than a normal period, or the cramping is far worse than you've ever experienced before, you should go to the ER to be sure you yourself are not at risk.
(Be prepared for a cold, impersonal experience at the Emergency Room, however*)
When in doubt, talk to a doctor. That's the best advice anyone can give.
