OP it completely depends.
I’m sorry, that’s really no help at all but every woman is different and there are a lot of variants.
Me -
pregnant at 37 first month trying. Pregnant again at 38.5 without trying.
I know a lot of women who had 1,2,3 babies in their late 30s and early 40s. In fact only two of my friends had kids before 30. The majority were over 35.
However it can’t be denied that fertility does decline, egg quality and quantity does decline. But also, are you having enough sex? I know that sounds daft but with two kids, is it happening enough? And at the right times?
Are you using OPKs to check your ovulation or measuring body temp?
There’s def lots of things you can do to try and improve chances of it happening.
38 isn’t super old, it’s definitely on the older side for fertility but it’s by no way an absolute that it won’t happen.
If you’ve been tracking ovulation and checking temps and having plenty of sex and it’s still not happening then maybe go private and have your day 3 and day 21 bloods done and DH has sperm checked - then at least you know what you’re dealing with. It wouldn’t be super expensive, few hundred. Not sure GP would do it as you already have children.
Good luck