It is perfectly normal for cats to have variable intervals between cycles. As my vet explained to me when I first brought my cat in over this, cats are induced ovulators which means they go into heat in response to a certain environmental stimulus. Usually in cats this is the presence of another intact male in the area (or his scent) but it also typical for the amount of light to effect this (which is why indoor or urban cats can go in and out of heat for the entire year). Based on a quick google search, most cats go back into heat after an average of 21 days, but I wouldn't know how a pregnancy would effect that. In my experience, a cat starts to cycle again (not necessarily jump back into heat) not long after weaning. But I also know from my experience of living with an intact female that the period between heats can be as short as 9 days to as long as 50 (this data all comes from the same cat). Since she is outside, then I would say her cycles are bound to be on the shorter side but without you previously keeping track of how she cycles in and out of heat, its hard to tell how long its going to be before she goes back into heat.
And without tracking her cycles, its also harder to tell when she's showing signs of being in heat (for example I can tell like about two weeks in advance when my cat is going into heat). But GENERALLY (I have read about exceptions to this in cats called silent heats) cats will show signs when a heat cycle is imminent (like within a couple days) and you should bring her back inside right away. These signs are rolling on the ground, being extra affection, and vocalizing more (its hard to say how much of an increase you'll see in these behaviors without knowing your cat, but I think you should find it obvious when she's suddenly way more affectionate than usual). Cats also go into a period called pro-estrus 1-2 days before being in heat, that's when these behaviors should ramp up. But more importantly, its when males will start being interested in your cat but your cat still won't be interested in sex yet. If you suddenly see more males around your house (especially when she's inside), males are following your cat around, and especially if your cat is around and the males are willing to stay near you, a scary human, to be around your cat (they may still flee from you, but they will want to stay within sight of your cat), then get her in right away. I'm not sure how true this is but it is but I did read this briefly once that one sexual encounter isn't enough to make a cat pregnant, so if you're alerted to her being in heat because you hear the screaming females often do after sex, don't waste anytime in bringing her back in. I will say though that as induced ovulators, each sexual encounter is likely to increase how many kittens she has, so no matter how late you are in bringing her in, its still worth doing so. And one last point, if at any time you pet her down the back and her hindquarters go up while the front part of her body is still flat on her belly, she is sexually active and its time to keep her inside (that position is called lordosis and is the sexual position typical of cats).
One final point I'd like to add because you talked about spaying her, its important not to be done while she's in heat. Her reproductive organs will be engorged in blood by then and there is an increase chance for complications. So if you're waiting for her to be in heat to know if she's pregnant or not, I'd keep her inside (maybe put her in a small room when you go out or set up camera by door to stop any escape attempts) until her heat cycle is over. If getting an appointment right after is a problem, then it might be worth keeping track of your cat's heat cycles so you can predict when she will or won't be in heat (always look for lordosis, that's a sure sign she's in estrus) to set up an appointment.
