There is no simple answer, it genuinely depends on lots of things.
Does he need your contribution in order to cover his costs? Did you agree to this ahead of moving in?
Do you get your own room or are you also sharing his bed ?
Do you make decisions jointly or is it very much his place ?
Do you see a long term future together ?
Might you have a family together?
Would you get married or is there some reason that they would never be possible ?
Who is named in his will?
How much income do you each have left over after bills are paid?
What lifestyle do you lead? Are you a trophy girlfriend who is expected to look glamorous at big social events , with all the maintenance that involves?
How are your incomes each likely to change in the years to come? Will you catch him up or even outstrip him or is that very unlikely?
How does half the mortgage on the flat or your usual proportion of bills compare to the rent you were paying before moving in?
I would be inclined to think that your fair contribution would be whatever rent you used to pay or would have paid. The apartment may be nicer than where you lived before, but would you have chosen to pay more yourself to live in a nicer space?
What you do with that contribution, explicitly put it towards the mortgage, give him rent, save it for your own deposit in future, put it into an account for joint holidays / fun spending or continue your punishing regime of hair extensions, diet and exercise and regular glamorous outfits
... would depend on your answer to these questions.
With hindsight, I would do whatever ensured that I got equal say in financial decisions.