is the relationship serious or not?
lots of good advice given above. find out what she WAS paying a month in rent.. it gives you the ceiling.
how my wife and i have it setup, she has her checking account, i have my checking account, we have a household account -to pay mortgage etc. i put in 1600 a month into the account, honestly, i have no idea how much she sticks into the account a month ~1k ish(i don't care as long as all the bills are paid). it covers all the bills & we have extra- i suck at paying bills on time so i let her handle that account (my checking account i do food, out to eat times, gas and my CC bills)
figure out how much in household bills there is a month (include taxes/house insurance unless that is rolled into your mortgage)
setup a house checking account for household bills - only- your total household expenses(utilities/taxes/mortgage/water&sewer etc. ) a month are probably ~2300 for everything a month. figure 2500-2600 a month has to go into the account- creating a buffer for unexpected expenses-
you put in ~2k a month - to cover the mortgage- your house, you paying (no issues)mortgage. if she was paying 1k+ a month in rent negotiate ~700 a month-minimum- that she is responsible for to go into the account- i have no idea where you live so i cannot know what the basic monthly rents are, in MY area 1k a month is considered middle/low on rent, in boston it is more like 2k a month in rent+ depending on which PART of boston you are in) just you have to make the amount Less that what she is paying now..
make sure you keep track of all expenses etc and then you will have more money a month to pay off CC bills and do stuff, she will have more money a month to pay off CC bills and do stuff.
just make sure you don't "dip" into the household account for personal stuff.
if you break up and she moves out, close the account, any remaining money in the account is split on how much money- as a percentage- she put in, say you both put in 3k a month on average for a the life of the account together and spent 2300 a month in bills, after a year the account would have 8400 in it ... if she put in 1k a month, vs your 2k a month, she would get back 2772 from the closed account, while you would get the remaining, if she -averagely- put in more or less, her % she got back- after bills paid would increase or decrease depending on what she put in.
if you get married ...you now have next egg to grow for vacations/kids/a life together.