There is only two places for oil to go. Leakage outside the engine..which you should be able to see on the floor or ground..or all over the case somewhere. Or burned- as in bad rings or valve seals. With bad rings it can still have good compression but oil can still get by and burned in the cylinder. Check the plugs-if that are not white or only slightly gray or tan, rather dark and oily, then it either has bad rings or bad valve seal at least on the intake side..which wouldn't be the first valve seals to wear out. If the plugs are normal-looking then it indicates it has either very bad exhaust valve seals or there is a breach into the an oil galley and porting to the exaust..likely a head gasket or cracked head issue. Just FYI, people that have continued to run the OE paper air filter on their 800s and 500s have averaged less then 1200 miles of dusty condition riding before their top end needed rebuilt. Those that have but rude in less dusty conditions average around 2000 miles. Also burning oil contaminates the O2 sensors and when it does it enough the MIL or Check engine light will come on. It also destroys and plugs-up the catalytic converter on the OE muffler. Yours sounds like she's going through a lot of oil.