I'm not the person to ask (I was there as a photographer and rope worker) - but my partner estimates decades. It depends on the extent of the damage. For the moment, it seems that the entire building will be gutted - and presumably structurally unsound. If it has to be rebuilt from scratch - look to the Sagrada Familia for a time reference..
The Sagrada Familia was attacked by anarchists during the Spanish Civil War, who destroyed Gaudí's study and the plans and models for the church. Decades were spent just reconstructing the project before actual construction could resume. In Notre Dame's case, there is much better documentation on the structure, so hopefully it won't take quite as long.
The latest news said that the structure is mostly saved.
Looking at some aerial pictures (https://pbs.twimg.com/media/D4OFOKMWkAEEHsz.jpg), it looks like the vault is holding except maybe for the section above the transept area (where the spire collapsed), but it has probably suffered, if not from the collapse of the roof, the heat has probably weakened it.
But yes, one or two decades of restoration is a realistic estimation, it's roughly what it took for other cathedrals to be restored (Rouen, Reims) after the World Wars (hopefully Notre-Dame will be in a better state after the disaster)
The Sagrada is not a good example as it is more complex than traditional gothic. Part of the original plans were supposedly unsound and several architects intervened over the decades. But granted, we're still probably talking decades.
