I used to run every day of the week. But the last 2 weeks I've been running only 3 days, Monday/Wednesday/Friday. Although, since I'm taking breaks every other day, I've been running much much harder on these days and I think the benefit is better for me.
The one thing that caused my shift is that one week in september, after a particularly great week of running, I could not run at all for 3 days. I'd try and start but within 10 minutes I was wrecked. On the 4th day, I felt much better but could only run 65% of what I was at the week before and I had to build back up from there.
I'm sort of playing it by ear though for the last 4 months, so who knows what I will be doing in october. (My workout is exactly 45 minutes / not counting stretching and warmup).