There are 86,400 seconds in a day. So just use days if the seconds are greater than that:
=IIF(Fields!TotalTime.Value < 86400,
Format(DateAdd("s", Fields!TotalTime.Value, "00:00:00"), "HH:mm:ss"),
Floor(Fields!TotalTime.Value / 86400) & " days, " & Format(DateAdd("s", Fields!TotalTime.Value, "00:00:00"), "HH:mm:ss")
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