If you fuse a Kyurem with a Zekrom/Reshiram, the level will be according to the Kyurem.
So say the Kyurem is level 80 and the Zekrom/Reshiram is level 50. When you fuse the Kyurem with one of them, the new form will be level 80.
If you're talking about Reshiram & Kyurem, then it's already been answered:
In Pokemon-White-2 do you have to train both Reshiram and Kyurem to level 100 then fuse them to max stats?
Im guessing you want to know the level of kyurem when you fuse him with Zekrom or Reshiram.
The level he is when you fuse em is simply the level of kyurem so its basically the level of kyurem, the level does not change. Only thing is the zekrom or reshiram that is being fused will still be the same level but you just cant see them as they are fused with kyurem, when you defuse them they will be at their same level before the fusion.
Hope I helped :)