If it is only using a quart in 1,500 miles, that's probably the rate it'll have for the useful life of the motor. If you hang on to the car for a long time, change the oil religiously and keep the oil level up, most likely something else will happen before the oil usage gets so high that it fouls spark plugs or something.
Keep these comments apply only if you are changing oil often.
I do have one additional concern though. You said you put oil in when the light comes on. I assume there is a low oil light, and your not talking about a low oil pressure light. How much oil do you have to add when this light comes on? Where is the level on the dipstick when the light comes on? Is this indeed a low oil level light, or a low oil pressure light?
Let's keep up this dialogue.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in