Long time
Presidents have been usurping authority for a long time. Did Kennedy have authority to send "advisors" to Vietname? If he did, Johnson almost certainly didn't have authority to begin *his* military buildup without a heckuvalot more Congressional approval. And we know Nixon did quite a bit of trampling himself. Bush, and by association Reagan (although there's no hard proof) is thought to have been involved in the Iran-Contra deals. And on and on.
Maybe Ford and Carter were the last two non-power grabbers? Ford, because he didn't last that long in office, Carter maybe?
The lawyers would mostly rather be what they are than get out of the way even if the cost was Hammerfall. - Jerry Pournelle