Hiya! I'm here now
It depends a lot on the school and what the general level of behaviour is like, as to quite how tough it is.
At interview I would want to know very clearly what is the average amount of cover I would do, and what is the maximum.
In my school I do 15 hrs a week cover, plus usually 2.5 hrs of form times/registrations. This is sustainable (for me in my school), and I wouldn't do more for the money I'm paid.
(Supply teachers coming in would do 25 hrs + 2.5 hrs - literally zip from one lesson to the next - I wouldn't do that.)
But also find out what else you will do with your time, and whether that will be enjoyable/rewarding for you.
I've been doing the job for almost 3 yrs, and it can be very hard at times, but it does get easier. And the students do learn to treat you differently to supply, because you are around long-term, and you do follow up on any/all incidents, and it's not worth their while to muck you about.
Having said all that generally I don't find 'cover' to be very rewarding - I think that's inherent in the very nature of the job. So I've fought quite hard to carve out some little niches/roles for myself elsewhere which give me some job satisfaction.