How soon is too soon?!

I think for July and any time during the school holidays - the earlier the better! People could book their holidays for summer way in advance if they aren't so close as to know the exact date of your wedding before the save the dates going out ?

We get married in October this year and sent ours in March last year as its October half term

It's normally about a year in advance but there's no harm sending them a bit earlier for peak summer holiday dates like July. I'd probably send them March/April time.