Hi Jazz,
I work as an ATCO at Frankfurt Approach control and as Denti already mentioned, we generally work inbounds alternating. This provides for a balanced use of the runways for both departures and arrivals. If you want to know early which one you will get as an inbound, try requesting 25L/07R. You will get it 95% of the time as tower works departures usually from 25R/07L. So with you requesting the southern runway, the worst thing that can happen for us is that we have two aircraft in trail for the same runway. But that's good for the tower because they have more space for their departures.
This said, my experience with pilots asking for the landing runway very early is as follows:
If I give the option of either knowing the runway 20mins before LLZ-intercept and knowing its 25L/07R or delaying the decision, thus getting a 50/50 chance for 25R/07L, most pilots take the option of a late runway assignment.
Cheers, Jürgen