The sentence does not really sound as legalese as it does slightly EFLese-- perhaps they are similar, though. I would guess that 'at all material time' means 'most of the time' or 'during a significant percentage of the total time'. The phrase, no matter how interpreted, however, seems inconsistent with 'constantly'.
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YankeeWhen used in a legal context, doesn't 'material' usually mean 'relevant to the matter at hand'? To me, this usage would be a legalese way of differentiating "all of the time" (i.e. literally all of the time, including the guy's time off, possible work elsewhere, etc, in the time frame of the renovation) from "all of the time during which he was physically present at