I'm considering booking two flights on separate tickets through Frankfurt Airport. How much time should I include for a transfer between the terminals 1 and 2?
I will be flying Schengen to non-Schengen, with luggage.
Just took one yesterday, and it took me 15 minutes.
If you use the free official airport shuttle running outside (which you have to do since you have luggage), it will take you up to 30 minutes, from the point you left the doors of terminal 1 to the point you entered the door of terminal 2.
The official stated transfer time is 7 minutes, but it only covers the shuttle driving time. The shuttle will not leave immediately once you arrive to its stop, and you might need to wait for the shuttle. It also makes intermediate stop on a bus station - usually nobody boards/leaves there, but if people are, this could add 2-5 minutes to your trip.
PS. Terminal 1 and 2 are defined by the airlines, not the flight Shenghen status. For example, Lufthansa flies from Terminal 1 both to LAX (Z gates) and to Hamburg (A gates). So if you're flying Lufthansa on separate tickets, you might not need to change terminals at all.
A relative of mine did this exact trip: transiting FRA from a non-Schengen country to a Schengen country on separate tickets, switching between Terminal 2 and 1. Gate-to-gate it took only 70 minutes, including passport control and the bus ride between the terminals. So if you're planning on making a similar trip, I'd say 2 hours should be plenty of time.