Without knowing what the internal temperature of your electric smoker is when it's set on medium, it's impossible to give you a good answer to your question. Smoking time and smoker temperature are two things that have an inverse relationship to each other - the higher the smoker temperature, the shorter the smoking time will be. A lower temperature will require longer smoking time.
Other factors to consider are the size, shape, and type of the meat being smoked. In your case, smoking a whole six pound Boston butt at 225°F would normally take around one and one-half hours per pound, more or less, to become tender enough to pull or shred (internal temperature of 190+°F), or up to 9 or more hours. Now if you just want it done enough to slice (internal temperature of 160°F), it could take only 4 to 6 hours.
The shape of the meat makes a difference, too. If you were to take that same 6 pound butt roast and slice it into 1 inch thick steaks, it would take less time to become fall-apart tender. But thinner cuts tend to dry out more if cooking to the shredding stage. However, if you were cooking the steaks up to 160°F, you'd save a considerable amount of time.
About smoker temperature, when yours is set on "Medium", you really don't know what the temperature of the smoke chamber is. (on a vertical electric smoker, you'd consider the smoke chamber to be the level at which the meat is...not at the top of the lid, where some smoker thermometers are mounted)
To have a better idea of how long it will take you to smoke meats, you need to find out exactly how hot your smoker cooks at the different settings.
To do this, you'll need a remote thermometer, the kind that has a probe that's attached to a display unit with a small cable. Get your smoker up to temperature, then place your pork roast in the smoker. Push the thermometer probe into the roast, through the top, and so the probe tip goes out the side. You want a couple of inches of the probe tip showing, but make sure it doesn't touch the metal grate, or anything else. Now you'll be able to monitor the smoker temperature right next to the meat.
After a short time, the temperature reading will remain steady. And that's the smoker temperature related to the setting your on. Adjust the temperature until it's in the 225-250°F range for most meats.
