How early can you purchase timed entry tickets to St. Peter’s?

Here's the website for reserving an entry time for St. Peter's. It's not the same one used for the Vatican Museums.

https://www.basilicasanpietro.va/en/products

The booking calendar now extends for one month, and I see tickets are readily available. Perhaps they've increased the number of online entries available for each time slot. The cost for a basic entry to the basilica (with audio guide) is €7. There are other options with add-ons at different prices. I didn't complete the purchase process, so I can't guarantee they won't tack on a booking fee, as the Vatican Museums do.

Booking procedures can change, so I'd recommend checking periodically to see if they've tweaked the booking calendar so it extends beyond the current one-month period.

We were just there. You do not need a ticket to get into St. Peter's. It is a working church, albeit the largest and most famous one in the world.

You enter the area via a road lined with souvenier shops and cafes. You will see St Peter's directly in front of you in the distance. When you get to St Peter's square you go through security. You will see a bunch of chairs in the middle that are blocked off by barriers. These chairs are for those who attend the Pope's Wednesday event. (also free--check the Vatican website for details) The line to get into the Basilica travels along these barriers. If you get there early, there won't be a line. We were there on a Saturday and there was a huge line. (We were on a Vatican museum tour and St Peter's and didn't have to wait in line since the church is connected via outdoor walkways from the museum) . You go into the Basilica one way and exit another--picture a clock and the crowd is moved counter clockwise in and out. (hint-- there is a ticket office-- I didn't pay attention to what it was for-but just pass there are restrooms--the official souvenier shop is one level above.) Once you are in the Basilica you are free to roam as you please or attend mass in one of the side worship areas.

We went on Sunday and saw Pope Leo's Angelus at noon. Our crowd had 15,000 people but it was fine. In July it will probably be alot busier and more crowded. But if you only want to see the Church, just get there early like at 7 or 8 am and you will most likely have no problems with crowds.

One of the advantages of the online reservation is that you don't have to stand in the general security line, but rather in a shorter line for those who've made an entry reservation.

Hi there
Not sure if this helps, but I was able to book two tickets for the Pétros ení Digital Experience with timed entry to the Dome including lift and entry to St. Peter’s (no guide) at the end of May through the basilicasanpietro.va website. I just did a quick search for just a visit to the Dome and St. Peter’s at the end of May and at the end of June and there are tickets with timed entry … was not able to search out as far as July so I am guessing that if you try two-three months out you may get what you are looking for.
Good luck!

You do not need a ticket to get into St. Peter's. It is a working
church, albeit the largest and most famous one in the world.

Just to clarify- yes St. Peter's is free. The OP is asking about the timed entry ticket now offered for a nominal fee (5 euros?) that allows you to skip the very long line and includes an audioguide.

We were on a Vatican museum tour and St Peter's and didn't have to
wait in line since the church is connected via outdoor walkways from
the museum

This entrance from Sistine Chapel is only available to select tour groups at certain times. The tour companies cannot guarantee this entry.