Further maths is another A-level, so it's however many lessons you have for any other subject I'm pretty sure.
In my sixth form that's 4 extra lessons (I take 4 subjects, and each subject is 4 lessons/week so I have 16 lessons rather than 12).
I don't do further maths but some of my friends do and it's just like any other A-level.
In terms of extra work being manageable or not I do know some people who have dropped further maths, it's pretty common - but honestly lots of people drop their 4th subject no matter what it is, and if you enjoy maths it's definitely worth considering taking it.
Maybe have a look at the course in more detail if you're looking at taking it in a specific sixth form. It might be good to see what topics there are and if you find them interesting.