Good question. I'm not sure he ever did stop entirely writing his own bars. He definitely has a lot more help now. From what I've heard, it was more around the time of MBDTF when he really started getting way more help with the pen.
People forget that not only did he write bars, but he did the no pen no pad thing. He used to put the beat on and just rap out loud in jibberish with a certain flow or cadence out loud, and as he'd keep rapping jibberish, he'd gradually fill in words to match the flow. There's some footage of this on a couple videos out there.
MBDTF is when he went full chairman of the board and CEO style in terms of flying out every rapper and producer under the sun. In reality he probably started getting more writing help a little earlier (maybe even Graduation era) but MBDTF is when he started just taking whole verses from people (specifically I've heard CyHi and Push wrote a fair amount of that album). And even then I think he wrote some verses on that album. Devil In a New Dress sounds like some shyt he'd write himself.
Is that where Jay came up with that bullshyt about him not writing pretty quincindental especially the way Jay was looking at him when he did the bounce.