Welcome to the forum, tony3542. (Curious about the number...) You'll find it is a fantastic source of information, advice, tips, and occasional (well actually lots) of weird humour...
Regarding the fan cycling time, it only very rarely gets to 70F up our way, so really have no idea. But I suspect that it will be far more useful to get the new temperature guage and then watch the relationship between heating and fan cycling. You should be able to see the temp rise, the fan cycle on, and then the temp drop a bit until the fan stops. Then you at least will know that the fan switch is responding to temperature rise and fall.
Good luck with your RTV! There are several others with 2005 models who I am sure can provide you with important and useful pointers.