The rules for Wild Magic Surge are:
Immediately after you cast a sorcerer spell of 1st level or higher, the DM can have you roll a d20. If you roll a 1, roll on the Wild Magic Surge table to create a random magical effect.
Are there any guidelines for how often this should happen? The player wants to roll for every spell cast, but the storyteller dislikes wild magic and hasn't had the player roll a surge yet.
It does seem kind of intentionally-vague. I suppose that's so the DM and players can determine how much wackiness they want to inject into the campaign. Perhaps there will be more guidance for this in the DMG, when it comes out.
Like most things that are left up to DM discretion, this is something that the DM and players should discuss ahead of time, preferably before anybody rolls up a wild magic sorcerer. It's important that the player knows what the rules are, even if they're totally arbitrary.
Some options that would make sense to me:
Rolling for surges after every spell cast seems a bit extreme to me, given the chance of a TPK for some of those results, especially for a group of low-level characters. On the other hand, the really nasty results are a small chance, on top of the small chance of surging in the first place, so unless you had two unlucky rolls in a row...
I like the second and third versions, especially if you can work them into the character's background and personality. Maybe they're more-likely to surge when someone they care about is being threatened, or when their hit points are low, or whenever they're affected by a condition.
Roll every time.
The exact wording says "can", but you aren't rolling on the wild magic table, rather a d20 to see if you roll on the wild magic table where only a 1 on the die will result in a wild magic table roll. Given that very low chance I think it behooves both the player and the DM to have the wild magic sorcerer roll everytime a level 1 spell (or higher) is cast.
The Tides of Chaos feature is another bag of cats though.
While the DM is able to force a roll on the wild magic table (vs a roll to see if there is a roll on the wild magic table) at any point after the player has used their Tides of Chaos advantage, it's probably only best used when it makes sense circumstantially. Why? because the DM controlled aspect of this feature removes player agency.
Yes, the player's handbook does give guidance as to when you should roll for wild surge. However it is very subtle and easily overlooked.
There are two abilities you gain at first level for Wild Magic. One is Magic Surge, and the other is Tides of Chaos. These two abilities are actually linked to each other, in that after each spell that you cast above level 1, you either apply Magic Surge OR If relevant, you apply Tides of Chaos.
What this basically means, is that every time the sorcerer casts a sorcerer spell of level 1 or higher, one of two things will happen. Either, you, as DM ask the player to roll a d20 to determine if you roll on the Wild Surge table, or you as DM, allow the player to regain Tides of Chaos and force them to roll on the table directly.
However, the rules are likely intentionally vague, which allows tables to come to the unintended conclusion, to be more accommodating to the tables that wish to reduce the amount of chaos that they have.
