The honest answer is earlier than most people expect, and the reason is simple arithmetic: a band can only play one event per night.
Here are realistic windows based on what we see across a normal year.
Weddings: nine to twelve months
Saturday weddings between May and October are the most competitive dates in the calendar. Nine to twelve months ahead is normal, and popular dates go earlier than that.
You do not need every detail finalized to reserve. A date and a venue are enough to hold the booking and start planning the vals.
Quinceañeras: six to nine months
Quinceañeras concentrate heavily in spring and summer, and they need rehearsal time for the vals and the surprise dance.
Six to nine months gives room to build the choreography and the music together, rather than forcing one to fit the other.
Birthdays and private parties: two to four months
These are more flexible, especially for weeknight or Sunday dates. Two to four months is usually comfortable.
Friday and Saturday nights in summer still fill early, so do not assume a small party is easy to place last minute.
Serenatas: one to two weeks
Serenatas are short, they are usually late, and they can often be arranged on short notice. A week or two is generally enough.
Same-week requests are sometimes possible depending on the night. It is always worth asking.
If your date is close
Call rather than wait. Cancellations happen, weeknight dates are frequently open, and a shorter two-hour booking is easier to place than a full night.
The worst outcome of asking is a no, delivered immediately, which still leaves you time to look elsewhere.