Here's a helpful link!
Basically, it suggests 2-3 months for a regular wedding, 3-4 months if it's at a special time of year (e.g. Christmas Easter).
As you aren't doing save the dates, I'd whack another month on, which makes it November.
If you're anything like me though, you've been screaming the date from the rooftops and everyone who needs to know does anyway!!
Personally, I would send day invites first with an RSVP deadline of January, then you have chance to upgrade evening guests if you have people that can't make it.
So, for example, mine is in September next year. I'll send invites out in April (we also aren't doing save the dates), with a deadline of May, then send evening invites out end of May/beginning of June.
Hope this helps! X