Considering that they are doing 90 minute episodes... it's the equivalent of filming a "standard" 6-episode British series... so it's not that much shorter than the kind of thing they typically do for a show, even a popular one.
I know Doctor Who gets about twice that many in a series, but they always say that it's like filming two series back-to-back from their perspective.
I'm sure he likes doing the show... As long as it doesn't keep him from doing other things that force him to make a choice, I wouldn't be surprised to see him keep doing it as long as everyone involved wants to keep making more.
One of the things that it seems the UK is a little better than the US at doing... in the US, if an actor is on a highly-rated show and the movies come calling... many TV shows won't let him do a movie until his TV contract expires and then they force him to choose. Think back in the day when Tom Selleck could have been Indiana Jones IF he could have been allowed the time to shoot it while on Magnum PI... but they wouldn't let him do it.
In this way, the BBC gets to have high-profile actors on TV series and some of them become even higher profile by going and making movies... then coming back to the UK to shoot more for the BBC because they like doing the show and it hasn't held them back.
The Doctor Who schedule, twice as much show, isn't as flexible... and has forced some actors to decide on more Who vs more lucrative offers elsewhere... and sometimes those offers don't pan out and then they can't just go back. Cumberbatch gets to be on a popular show that has a shooting schedule that allows him lots of free time to make other stuff. And we all are better off for it!