Does a Wild Magic Surge trigger at the beginning or the end of the casting of a spell?

Question

This is, in some way, a follow-up to this question: Can you trigger Wild Magic Surges with rituals?

Some spells take longer than an action to cast (the best example is a ritual, which takes 10 minutes).

It is possible, as a Wild Magic sorcerer (PHB p. 103-104), to have access to sorcerer spells that take longer than an action to cast (notably, by taking the Ritual Caster feat and choosing the sorcerer class).

My question is: if the DM agrees for a Surge to happen upon casting a spell with a casting time of longer than an action, when could the Surge happen; at the beginning (first turn of casting, therefore allowing a Surge even if the long-time casting is somehow interrupted before its completion) or the end (last turn of casting)?

The ability says "immediately after you cast a sorcerer spell of 1st level or higher", but if I'm not mistaken, for longer casting times, you must take the "Cast a Spell" action every turn, hence why the wording makes me wonder when the Surge could happen.

Answer 1

At the end of the casting of a spell

The important thing is the quote you mention in your question:

immediately after you cast a sorcerer spell of 1st level or higher (PHB, pg. 103)

It says "after you cast", so any time before that you are casting the spell, and only once the spell's casting has been completed have you cast the spell.

Answer 2

After the spell has been cast completely

As you noted, the language from the PHB(Page 103)

Immediately after you cast a sorcerer spell of 1st level or higher

You can also reference Chapter 11 Spellcasting rules (PHB, 202):

Most spells require a single action to cast, but some spells require a bonus action, a reaction, or much more time to cast.

...

If your concentration is broken, the spell fails, but you don’t expend a spell slot.

This suggests that a spell has not been cast until has completed casting.