I love it! Thanks!

We played 4 nights per week until last week and were up to vashj in ssc and alar/VR dead in the Eye, from 19:00-23:00. We could clear all those places (including gruul/mag) in 2 raiding days and spend the other 2 at the new boss. Recruiting is key if you want to follow a casual/serious approach, I think our progress was relatively (to time played) quite good because we made sure we had people who wanted to play wow no more or less than 3-4 days a week and also we made sure we didn't keep people who were obviously incapable/lacking understanding of game mechanics in a profound way. We recently added 1 more day more because we have a pretty large playing base and we wanted to make sure everyone gets to play more than 2 days at least.

We had cleared kara 20th february or so and took it out of the raid schedule early april. There's still 1-2 groups running it in offdays, by anyone who wants to and it seems to work well. When to drop raid instances is a matter that was bugging me since the old days when my old guild totally failed to move into naxxramas as bwl/aq took all our raiding time and people were not motivated to cut down time on them. I think the best time to cut it off is when you are really sharding more than 60% or so of the loot and people are rather bored of it. Also, hydross is even now the biggest dps/tank gear check in ssc, so if you can actually kill him there's just nothing you need from karazhan anymore really. But if you are still getting people who are not trialists or so attuned, you do still need it.

I am in a guild very similar to what you describe. We raid 3 nights pr week from 20-23 and have recently killed Magtheridon. What works for us is to have a fairly large rooster (enough to support 40man raids actually) and have people play when they can while the raid-leader tries his best to put the people that show the most dedication and will to progress into the raids where that is needed while new people or people that come back from a break get to show themselves in Kara.

We still maintain kara on our schedule, usually running 2 or 3 groups in addition to having one purely casual group, or light group as we call it. Having Magtheridon and Gruul on one night, this gives us 1 night of attempts at new bosses. How I see it, the moment we are stalling progress on our 3rd night because of lack of time (too much time doing content we already know so we get limited tries in on new stuff), it is time to cut Kara as an organized raid.