My experience was that it will just get worse from the point you are at. The freeway speed temp will slowly drop over time and of course especially in cold weather. Only when temps get down to somewhere in the 60s when cruising will the plausibility code be set.
Don't wait -- just get the tsat changed along with other associated things (temp sensors, hoses, etc). There is nothing good done to an engine that is slow to warm up and operates below design temperatures. 75 isn't "awful" yet, but much below that and trouble is knocking on the door just waiting to be let in.
+1 Just get it changed. There are two ways the thermostat can fail (my indy gave me a detailed explanation of the two ways which I can share if anyone is interested); one way throws the 69 (obligatory heh heh) code but the actual oil temp gauge does not change too much from normal; the other way does not throw the code but the temp gauge will show sig lower temps, often at or below the first dot.
I just had mine done due to the code. I've been expecting it as I am at 70K miles, which appears to be the average life expectancy of the thermostat on our cars...
2001 M5 LeMans Blue/Silverstone Dinan: springs, Konis, rear sway bar, monoball bushings, and exhaust;
Ground Control camber plates, Stoptech front BBK, Brake ducts opened,
Evolve AlphaN tune,
OE 9.5" rear wheels all around & 275/35-18 Pilot Super Sport
2011 M3 sedan 6MT Silverstone, no sunroof!
SCCA F-street autocross device: Dinan front swaybar, pins-out-max-camber-up-front, BFG Rivals/Michelin PSS/Nitto NT-01
2006 330i sport 6MT Electric Red/Black
2001 Audi S4 6MT -- for sale?
1996 Volvo 850R wagon
1975 CanAm 125MX2
It took me personally about three hours the first time I did it. I am somewhat mechanically inclined and that was my first T-stat change on my M5. I knew it would have to be done when I saw the temp gage low during a test drive. I didn't price the job until after I was the owner. My indy said about $350 so I decided to try it myself. Not so hard but I did notice it may have been alot easier by removing the plenum. I noticed how more accessable the housing tubes were when I renewed my tank vent valve. Also, if you had a leak at the tube mating point it would be easily detectable after refilling. Three and one half hours seems a little high if he is experienced in doing this job. If he is not experienced, I would consider finding someone who is.
Just replaced my thermostat and temp sensor on the top of the housing yesterday, not too difficult. I took the advise of many here and used some lube on the O rings, went in pretty easy. Previously the coolant temp was 73 tops on the highway and the temp needle was always parked on the left dot. Now the temp stays at 79, and the temp needle is just a hair left of dead center, very pleased.
'Tis the season apparently (really we all just notice the t-sats that are just beginning to go out in the cold weather). Ordered mine last week as it is like several folks have described...floating around 74/75 on the freeway. Will EVENTUALLY get a bit warmer, but always sits well to the left of center in these Winter temps. Ordered a couple extra o-rings as other have recommended. Who knows, maybe it will even run a bit better afterwards.