I'm sorry you're in this situation, it's hard.
But 5 year olds can be really accepting and adaptable.
I told my 4.5 year old one month ago today that her father and I were getting divorced, what that means (living apart, don't love each other) and took her to see my new house the same day. She's been really excited about the house, and totally accepting of splitting her time. In our case, helped I think that I've always worked away a lot and then we've lived very separate lives at the weekend.
But also helped that we've both talked about the split positively.
Before I experienced this, I'd have been the first to talk about rules and timescales for meeting new partners. But now... I really wouldn't worry. She knows what a "date" is (thanks, Disney "Enchanted"!). Yesterday she said she wanted to be a bridesmaid so I took the opportunity to throw in that if (I did emphasise IF) daddy and I got remarried she could be. Reaction? "Then you mustn't get married on the same day and oh! wonderful! I could have 2 mummies and 2 daddies!!!"
I think you have to be careful how the new partner is presented - e.g. It would be confusing to present them as a permanent new stepmother rather than a dating girlfriend.
But actually, I would now introduce my 4.5yo to a new partner much quicker than I ever thought I was.
Frankly, she doesn't care that we're separated! Which is a good thing.