Almost certainly within the next 6 months or so.
Why? Well, first of all, there will probably be enough sales by then that some people will find themselves with points that they can't use. So their choice will be to Bank or Rent. Riviera will probably rent at a relatively high price, so Renting them out might be attractive.
However, in addition, almost anyone can reserve Riviera and rent the points to do that in the 0 to 7 month window. So far, that window looks tight, and truthfully, in the long term I expect it to remain very tight. BUT, there will be various times when Disney 'Declares' new units. Don't know how soon that will be. Probably about the time that they start to get low on what they have already declared.
Riviera will have almost 7,000,000 points and they have already declared units sufficient to account for about 1/5th to 1/4th of the total points (I think). Assuming it will take approximately 5 years to sell out, that means that they will probably make new 'Declarations' about once per year. Whenever they do, then new units will become available to reserve, and that means there will also be new points out on the Rental market, trying to use those points.
So far, here are the number of units Declared, out of about 300 units total:
The first declaration includes all villas on the 5th floor along with the westernmost wing of both the 6th and 7th floors.
- 9 Two-Person Tower Studio villas
- 8 Dedicated Deluxe Studio villas
- 7 Dedicated One Bedroom villas
- 18 Dedicated Two Bedroom villas
- 23 Lockoff Two Bedroom villas
- 4 Three Bedroom Grand Villas