I get a sneaking suspicion this question has been asked at least 20 times in the last 12 months.
Ok. You've had successcwith one before. How thick was it? The same 3 inches even though it was a lighter piece if beef? Remember we are talking heat penetration here and the time it takes to reach thermal equilibrium. The only thing that matters in calculating that is the diameter of the meat tube at its thickest part. It doesn't matter if that tube is 10 inches long or 10 feet (imagine the monster cow that produces a 10 foot tenderloin.... drool). 3 inches will take about 3 hours, so let's make it 4 to allow a margin for error (and the fact that our 1 hour per inch doesn't accurately reflect the thermal absorption curve).
Tenderloin, being the least worked muscle on the beast, is naturally tender and needs little more than coming to equilibrium.
If you think the full piece is too big for your cooking vessel, cut it in two. It's much better to do this for ease of handling when searing anyway.