It all depends on you really... You are expected to do 180 days away in a year, though in truth it seems to work out more around the 200 mark for most underwater engineers. You can ask for more days if you like, and the more days you do, the more you get paid per day, it goes up in brackets, and you get your basic salary on top. But yes, my work pattern thus far has been:
4 weeks away training
6 days at home
8 weeks away
3 weeks at home
I'm now on another 8 weeks away
But I WANT the extra days because I have debts that need shifting and new outgoings that need to be catered for, so a year hitting it hard is the order of the day for me.
When I applied and got offered the job, it was actually a few months before I started working for them, because the RAF wouldn't release me when they promised and decided to arse around instead. So, if you are interested, it might be worth writing up your CV and winging it off soon and seeing what happens. They seem to be happy to wait for the "right" people than to quickly employ the wrong ones.