Entirely depends on your riding style and conditions.
Are you riding mostly around town? Back roads? How heavy are you? What's the temperature outside? Road conditions? Etc., Etc.
I ride @ 38 rear, 36 front on my SS around town commuting and have never run into a single issue, same tires, same bike out at the track, I'm at 26-27 rear, 27-28 front.
I also get about 8k miles out of a tire when only commuting. And have thus far gotten 8 track days + some commuting out of the tires I have on there now (Pilot Power 2CTs) and still have plenty more tread left. Will likely last me the rest of the year (3 +~1 more track days).
A flat number as a recomendation without some sort of explination as to what kind of riding you're going to be doing doesn't make a whole lot of sense to me, sorry.
FOR SALE - 2002 900SS: Galfer Wave rotors + Carbon front fender + Ducati Performance levers & high mount carbon cans + Cycle Cat clip-ons & triple tree + Evoluzione Slave Cylinder + Motowheels Slipper Clutch + Power Commander w/ custom map + Air box mod & K&N + MBP Touring Wind Screen + Penske front fork rebuild