how long does it take for lower abs to develop muscles?

[FrostByte]

I've been working out 4 days a week for about four months now. I do ab exercises and 5 to 6 miles of cardio each time, as well as alternating a different muscle group each time. My lower abs are stubborn. How long does it take to get the fat off of them and muscular looking? I'm mean they're not real fat. I'm 5'10 and 164 lbs so I'm not overweight, but there is some fat on the lower abs that I'm having trouble getting off. Any tips on this?

Thanks

 

[g8r80]

Lose the fat that's covering the abs. If you get lean enough, they will be exposed, but exercising the abs themselves will not do much to make them visible.