Normally 2-3 hours. Like to avoid queueing too long for security checks and getting there early in the day normally helps.
Also, I've just got a Priority pass for 500 airport lounges worldwide inc. 3 or 4 at Heathrow and Gatwick, also at Stansted so we'll shall be taking advantage of that!! (It also includes McCarran so at last we can relax a bit more when in Vegas)!!

TC