Unfortunately, there is no time limit to receive a decision. However, you may apply for naturalization as soon as you have been a permanent resident for 4 years and 9 months. If the I-751 is still pending at the naturalization interview, the Officer should adjudicate both at that time.
It depends on the facts and circumstances of each case. If your case is pretty straight forward, it should not take that long. If they think there is an issue, you know the answer. It may take long.
609-271-3573. This answer is for informational purposes only and should not be construed as legal advice.