How long is a piece of string?
It depends on the fleet, base and season.
On the FBW which almost all new recruits are taken on for, you can expect to be busy in the summer 80-90 hours. Less so in the Winter plan on 40-60 hours.
Any voyage on these fleets is likely to be short term voyage to another UK base. There will be some, but it shouldn't figure too strongly in your fiscal planning.
Duty pay is taxed at your highest marginal rate on a fixed proportion of the amount received. Flying pay is fully taxed at your highest marginal rate. Salary is taxed in accordance with your own personal circumstances at the published rates.
Non FBW fleets (757/A300) can generally expect much lower hour utilizations in both Summer and Winter. There may be greater opportunities for voyage pay, but this is more than offset by the lower proportion of duty pay and flight pay resulting from the lower annual utilization.
Everybodies take home is going to be different, depending on their own circumstances, but these generalizations together with your own figures should help you do the math.