Paris River Tour Reservation (without meal) how far in advance to book?

Why do you want a reservation? There are two companies with huge tour boats that have departures every 30 minutes in prime season, April to September. Bateau Mouches on the Right Bank between Pont des Invalides and Pont de l'Alma, and Bateau Parisiens on the left bank at the Eiffel Tower. Just walk up and buy tickets to which ever one you want and get on board. The tours follow the same route, returning to where you got on, with multi lingual narration, and last a little more than 1 hour.

There are also smaller companies along the river. Maybe you would need a reservation with one of them, but you did not specify that in your post.

In addition to the river cruise companies Sam mentioned, there is my favorite, Vedettes de Pont Neuf. As the name implies, their dock is at the end of the Pont Neuf, close to Notre Dame, on the Left Bank. In all our visits we have always just walked up and bought our tickets for the next cruise.

We've tried several companies and Vedettes de Pont Neuf is by far the best in our opinion. The trips at night when the city is all lit up are really impressive. We never made reservations and never had a problem getting aboard.

I'll just add that if you are going with Vedettes du Pont Neuf, you can get a €2 discount if you book an online ticket good for any cruise, and a €4 discount if you do book a specific departure. But I agree that there's no need to "reserve" a non-meal cruise, since they depart frequently.

To see discount offers, click here and scroll down: http://www.vedettesdupontneuf.com/home/

And I once bought a discounted ticket from my phone about a half-hour before the scheduled time and just walked up to the Pont Neuf docks with the receipt on my phone.