Hi Cameron, and welcome!
Among the best tools a pitmaster can have are good, accurate thrmometers to measure both cooking temperatures and internal meat temperatures. Many members here, me included, use the Maverick dual probe, remote electronic thermometers. Tjohnson, another member, sells these at A-Maze-N Products (link at bottom of page). The key to a successful loin smoke is monitoring the internal temp (IT) of the meat. Loin is a pretty lean cut of pork, and can be dry if it is overcooked. You want to shoot for a finished IT of 145-150*, then double wrap the loin in foil and let it rest for about an hour in an ice chest or a warm oven. The last loin I smoked was not quite as big as yours, and took around 3 hours to finish @ a smoker temp of around 240*. Just remember that most folks here will advise to cook to a temp rather than a time, and that most time estimates are only a guess because each piece of meat tends to have a mind of its own.
BTW, it looks like this is your first post. When you have a chance, stop over in the Roll Call Forum, intrduce yourself and tell us a little about you. It'll give other members a chance to get to know you a little and to give you a proper SMF welcome.