I'd go with a set rate, like salary versus hourly, that way it isn't such a formula that you have to adjust - if the kids don't fall asleep right away, sleep in, get up early etc. Figure approximately what it should be worth to you and to the sitter, who hopefully knows you, your kids and your house, and talk a lot before hand. Because it is a big chunk of time, you should both be happy with a bulk rate. Communicate what you want, what you don't want, what you will provide and if this goes well, maybe it could happen a few times a year, which gives both of you incentive to make it feel like a fair price all around - if it isn't too expensive maybe you can do it more frequently, and if it feels like a fair price your sitter may be willing to do it every 4-6 months or so. Have a set rate in your head, then ask what they think is fair, then see how close you are to each other's number and compromise. The key is communication and trust.
Remember that taking time for you makes you able to be a better mother; and the best gift a parent can give to a child is to love their other parent. You are taking care of you and your relationship - what a great gift to you, your husband and your children! Have Fun!!!!