How to wire a bathroom fan with a timer?
There's no such thing as too many wires between the bathroom switch and the bathroom light/fan/heatlamp/whatever the future may bring. I would run at least 3 hots and a neutral, with 1 hot and 1 neutral being 12ga for a high current device. Even better, install conduit and pull whatever as needed. 

I personally hate fans which turn on with the light.  I fear I'll wake others during a 3AM bathroom run.  It also wastes energy by ejecting nice warm/cool air, forcing the HVAC system to work harder. 

This forum is not for "shop for you", but there's no problem getting a single switch that turns off the fan some period after it turns off the light.  the fan with a delay. on light+fan, and turns off light and adand continues the fan for a designated period.  

There's also no trouble getting rundown timer switches if you want to manually trigger the fan.  However if you also have a heatlamp (commonly put on rundown timers), this will be a confusing UI.  

They even make fan controls which detect humidity.  The question is whether they have, um... VOC detection. 

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