According to The Wrestling Observer Newsletter, Brock Lesnar’s last WWE dates for wrestling on his current deal would be the upcoming WrestleMania, or the television the next day. There is an option built into the contract that would renew, and most expect that it will be renewed. WWE also has certain rights with Lesnar that will go through the first week of August 2018 if a new deal is not signed.
In regards to Lesnar returning to UFC to fight Jon Jones, the feeling is that the fight wont happen due to Jones upcoming USADA suspension. If Lesnar does want to do another UFC fight, if they wait until August, that would be the cleanest way to do so since WWE would have no rights to him. Lesnar felt the deal last year, where he wrestled through Mania and fought in early July, left him too little time to get into top shape. He would rather have a four-month window as far as getting back into that kind of shape before a training camp.
Sources say that even though WME IMG, the owners of UFC, represents WWE, things arent smooth between the two sides. Dana White has a good relationship with Triple H, but feels things are more difficult working with Vince McMahon. Unless WWE & Lesnar agree to a new deal (which is expected), he will be done at WrestleMania. If he wants to fight again, the earliest would likely be August 2018.