I guess the amount of practice time as an EMT would depend on the type of service you can get in to as an EMT. A slow paced, low volume service might require more time, where as a fast paced busy 911 system will afford alot of experience in a short time. I have always recommended at least 1 year to "get your feet wet" while under the proctorship of an experienced partner, preferrably a medic to offer guidance and education.