Can you be any more specific? You're talking about side milling? Are you doing deep pockets or something? Can you rough with another tool to remove most of the material? Are you using a CAM program?
I'm awaiting opinions from the more experienced guys but my belief is that you can do it if you take many, many shallow cuts. In Mastercam, it would be 'Depth cuts'. I'd do a finish pass 'on every level' and I'd set the max depth to be 0.040" or something in that ballpark.
It's going to generate hundreds of rough and finish passes but that's the price you pay with a long mill like that. That's why I'd start by roughing most of the material with something bigger.