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.