There is no one answer that everyone agrees is fair. You have to figure out what works for you - I can tell you what works for us and that we feel is fair for us, but some other MNer will be along shortly to tell you the opposite!
With that caveat, when we were young and starting out I earned £3 to DH's £1 so we added up all our joint expenses (rent, utilities, spends on shopping, sinking fund for fun things together like restaurants and holidays) and contributed to that in a 3:1 ratio. So if our joint expenses were £2k a month, I put £1.5k in and he put £500. The balance of our salaries we kept for ourselves. Over time, when DH would get a raise so we were earning the same, we would recalculate the ratio to contributing 50/50, when I wasn't earning for a bit on mat leave we lived off his salary, etc.
Once we had kids, we did it a different way - now we keep a set amount of money for ourselves, put a set amount into joint savings, and contribute all the rest into the joint account out of which goes the mortgage, bills, food, kids clothes, etc. Anything left in the joint account at the end of the month goes into the joint savings.