The 1st level wild magic surge class feature says the DM can have you roll a d20 after casting a non-cantrip with a 1 leading to some wild magic effects. Would you require this roll after every spell as DM? Do you think it's intended to be used sparingly?
Similarly with the 1st level tides of chaos feature (you can gain advantage once per long rest, unless you cast a spell and the DM makes you roll on the wild magic table, in which case you get another use). How often would you ask for the roll and "recharge" the power? (From every time to never). Would you let it recharge multiple times in one period between long rests? What do you think was intended?
In the spirit of the random nature of the feature, I would probably make a d20 roll whenever the Sorcerer cast a spell and use the result to determine if they needed to roll for a potential Wild Surge. Depending on how frequently I wanted to see this take place I would use a target number of 10, 12 or 15 as the "DC" to determine if a roll was called for. I think I would also add the level of the spell just cast by the Sorcerer so that higher level magic would be more likely to potentially trigger a Wild Surge.
As an example, if I choose to set the DC at 12 and the Sorcerer casts a 3rd level spell, I would roll a d20 and if I rolled a 9 (+3 for 3rd level spell = 12) or higher on the d20 I would have the player roll for a possible Wild Surge. I think it might also be good idea to have a Wild Surge take place on a number equal to or less than the spell level cast. So, a first level spell would only call for an actual roll on the Wild Surge table if the player rolled a 1 on the d20, while a 5th level spell would do so on a roll of 1-5 on the d20.
I would simply proc it when it would be dramatic. 5% chance after every non-cantrip probably means you'd be rolling a lot of d20s that don't do anything but waste a few moments.