Tough question to answer without more information......
Lots of factors involved, such as how often have you changed your oil? Have you ever had valve seals and guides replaced?
If you have original seals and guides, and have changed oil often (every 3k miles) then one would expect to burn 1-2 quarts every 5k miles or so.
IF you are burning more than that, and/or you are seeing blue smoke from tailpipe, then it's time to change the seals/guides.
'91 300E, 208k miles