My characters, if they're spellcasters (they usually are), have set at least four spells (a Ward, a Veil, a Sight-Blinder and a Summon Landmine), each one at the power they can reach at their Base Complexity + 1 skill declaration per apex skill + mild consequences. Effectively, my serious protections can be done at 1 casting scene and 1 rest scene per spell. For a submerged spellcaster, that usually is; 5 base complexity +4 from a Discipline and Conviction skill declaration, +4 mild consequences = 13
This is the effective baseline a submerged spellcaster gets if he is not in a terrible hurry but doesn't want to spend more than one scene as time resource per spell. Do note however that serious protections usually follow a day-long casting time rather than scene-long. Given the slowness of thaumaturgy scenes, these would be 8 scenes (one scene per hour) for a day-long preparation.
This gives 5 base complexity, +16 from mild consequences (2 milds every other scene), +16 from skill declarations (2 per skill of Great or Superb, but no more than one per scene), +2 from fate points, +4 from moderate consequence, for a grand total of 43
That is the upper limit of what a skilled submerged wizard should be able to do because that represents the work of 8 hours. If you don't allow interruptions in preparation for sleeping, eating and the like, it also comes as a hard limit; they shouldn't be able to amass more power any faster without better skills and more power (i.e. consequences due to high conviction)
