Rats fairly clean creatures, they will only stink as much as you let them. Compared to mice their urine isn't all that strong. I keep mine on kiln dried pine and change the bedding (and rinse out the tubs) once a week every Saturday morning. I normally don't even start to smell them at all in the room until cleaning day.
The biggest factors I can think of on how long it will take rats to start smelling is:
- The type of bedding being used
- How big the room is they are being kept in
- How much ventilation is in the room
- The average temperature of the room
- Age and number of rats per enclosure
You may just want to play it by ear (or nose) and see how long they can go in your particular setup until the bedding produces a noticeable smell and plan your cleaning schedule accordingly.
But honestly I think if you keep your 1.2 on pine and clean twice a week I doubt you'll smell them all that strongly, if at all.
Like I said before rats do not produce a smell nearly as bad as mice, but if you still find the smell unacceptable you may want to consider ASF's as I've heard their smell is even less than that of a rats.