Hello @Cleahy and a warm welcome to the Forum 
There are plenty of sitters who like to plan in advance so it’s certainly not too early to post a sit now for September 2024. Of course you will always have other sitters who prefer to secure their sits nearer to the dates. Sitters have many different reasons for how far in advance they book their sits, some would consider this too far in advance.
I have posted another thread you might like to take a look at by clicking here