IF the temperature rise was continuous, you would be correct. However, the temperature increase is not a linear function with time since there are all sorts of heat loss paths that reduce the rate of increase as the temperature increases above the ambient value until those losses match the power input. Then the temperature approaches a maximum. This happens asymptotically, that is, the rate of increase continuously diminishes to zero at infinity!
Most preheaters can be left on indefinitely and use of a cover will give you a higher temperature than using Cowl Plugs. My interest was to find out how little lead time would be necessary.
Many articles suggest that the engine should be at least up to 40 degF to avoid unnecessary wear. This is often used by flight schools as the point to start using engine preheaters. Also, here in the Minneapolis area, most GA flying grinds to a near halt at 0 degF so I was not interested in temperature increases of much more than 40 degF. In either case (with a Cover or a Plug) this is reached in 3-3.5 hours. Therefore I did not present any data beyond 4.5 hours.
My planning is based on getting the engine to 45 degF. In either case, the nominal rate of rise for this range is about 15 degF per Hour. So, if the ambient temperature is 0 degF, I ask (or otherwise program) that the plane be plugged in at least 3 hours before flight time; this would be 2 hours ahead of time for OAT of 15 degF and 1 hour for 30 degF.
As mentioned above, you can leave the heater on a much longer time, I rely on the heater suppliers to not burn up the plane. I was most interested to find out, HOW SOON CAN I FLY? For a short time application (e.g. 3 hours), there is no significant difference between using a Cover or Plugs. I find the Cowl Plugs easier to manage.