Just like my 2.0TS 75, my 1.8TS 156 uses loads of oil, no idea where it goes and sometimes I have to tow my white box trailer to shows - not a drop of oily exhaust residue on the trailer.
I check it every week, probably use 1l a month (1000 miles), but more if towing. Doesn't help that the dipstick is so hard to read.
I wouldn't worry about it too much mate, just keep it topped up.
As I said in a previous post, my old 155 V6 used to drink the stuff, hell it used to have an oil pressure gauge, two oil pressure lights and an oil level light that used to come on when the level got too low. I used to have to top it up every week as it was easily using a litre every 1000 miles.
I had the car for over 3 years, the engine never faultered once! I never saw blue smoke out of the exhaust, I even got my mates to follow me (or try to LOL) to see if there was any blue smoke out of the tailpipe under hard acceleration but nothing! The only thing I used to get was soot around the bottom edge of my tailpipe and to be honest I get that on my 156 TS which has just had a complete engine rebuild (and uses oil).
The problem I have is getting back into the habbit of checking the oil weekly! My last car was a Toyota Celica GT which never used a drop! (well that and trying to get a decent reading on the dipstick, which seems to be a pain in the ar5e on the 2.0L TS!!)
At the end of the day, I've just resided myself to the fact that it must be a characteristic of these cars! I don't have to worry about changing the oil at recommended intervals, mines on a constant oil change!!