I have twice had a long term, living together relationship, both times we arranged it the same way.
Start by putting together a budget of what will need needed to run the household, rent/mortgage, utility bills, food, etc. Doing this, means that you agree what will be covered, and what, if anything, not, may be phones, cars etc, whatever works for you both. Set up a joint account, consider whether it needs overdraft facilities.
Now you know what household money you will need, agree how to split this. If you both early roughly the same, 50:50 seems reasonable. When we have very different incomes, we paid into the household account in proportion to our wages. Everything else is our own to save or spend as we see fit.
If/when things changed (new job, pay rise) we adjusted the percentages. Nowadays, when we are old gimmers and money is less tight, the household has been known to take us out for meals, on holiday etc. It gets fed extra in anticipation of Christmas.
This works for me, can't imagine doing any other way. I keep an eye on my own account, and automatically rein in if I notice that it is getting low. I wouldn't feel happy having to rein in my spending because someone else had spent more than usual.