This is from Ngaire's website. Could find nothing on Regent's, strangely.

How many formal, informal and casual nights will there be on my cruise?
If a cruise is not termed "Country Club Casual" then you can count on a mixture of formal, informal and casual nights. Although cruises can vary slightly depending on ports of call and areas of the world the following is usually accurate
7 night cruise: One formal night, and then an even mixture of informal and casual
10 night cruise: Two formal nights and then an even mixture of informal and casual
14 night cruise: Two to three formal nights and then an even mixture of informal and casual
21 night cruise: Three to four formal nights and then an even mixture of informal and casual.
21 plus nights: These will usually be in "segments". Each segment will follow the guidelines above so you can count on one formal night each 7 - 10 days.

From my memory, the non-formal nights are not evenly divided, rather tipped a bit in favor of cc casual.