I think a 690 is ~23000 rpm. 30ipm is way too slow to get good tool life at 23000 rpm. 100-150ipm would give you better tool life. MDF is extremely abrasive and dulls bits quickly at any speed. In a production environment, if i was cutting MDF non stop for a full day, I'd probably use a new bit every day.
If 30ipm is your max, you could try to find a single flute spiral, which should last a little longer, but probably won't be cheap. I think the most cost effective bit for you would be to use carbide tipped straight bits like these.
http://eagleamerica.com/straight-bit...ng/p/102-0102/
One thing that may also help is to clean them after each use with a bit cleaner. Keeping them clean will reduce heat, which is a major cause of dulling.