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They are still fully synthetic and not subject to contamination by combustion products so won't degrade at a significant rate. If you're subjecting the transmission to heavy use you want the fluid to trap the wear products and hence it's a good idea to change the fluid more frequently.
But otherwise in normal use they are good for more than the 2 years of a non Life Time Fill fluid.
As I mentioned elsewhere, I changed the Life Time Fill fluid on my superannuated E46 at 150k miles and it made a significant difference to shift quality (the colour of the stuff that came out was soot black), but it was still working - and continues to do so some 5 years and 30k miles later using just regular MTF fluid changed every couple of years.
Once more, HTH.
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