Very long time for heat

I have a 2003, 2500HD with the Diesel. Since it finally started getting cold outside, I have tried the heater the last week, and it takes about 7-8 minutes to get any real heat in to the cab. The local dealer says that is normal since it is cold outside (even though the temp outside is only 40-45 degrees). Anyone have any insight if this is how long the heater should take to warm up? 7-8 minutes of driving at 40mph seems too long...especially in this not-so-cold weather. Thx, Scary L

If you are talking about 7-8 minutes of run time it sounds normal, if you are letting it warm up for 10-15 minutes and then taking off and 7-8 minutes later it starts to heat, it may be a sign of a weak thermostat.