Depends on the cop. Many police officers are men and women of their word, but some are not. The best policy is to have an attorney negotiate with the police and the district attorney and get the agreement in writing. Many times the police exaggerate the trouble you are in or the evidence they really have.
Many people wind up getting strung along as the police keep asking for just one more piece of information and it never seems to reach a conclusion. An agreement should include exactly what benefit(s) you are to receive, what they are expecting, who it is they are trying to arrest, and how long or how many times must you assist. These things should be explored before putting yourself at risk as an informer.