Hi Leo,
Just be aware that the number of years is likely to be quite small. According to the statistics published on the RIPE NCC's FTP site, it assigned more than 1,000 /24 prefixes last year.
We are talking about PA prefixes here, and this pool is only meant for initial allocations (PA), not PI.
The same statistics indicate that about 3,700 prefixes of all lengths were assigned or allocated last year. I don't think we can assume that demand for IPv4 address space will reduce and I also think it is reasonable for networks that would previously been happy with some PA space from their ISP to get space direct from the RIPE NCC if it is the only game in town.
True. There might be organizations that become an LIR to get that initial /24 allocation.
I don't know how many years people want small blocks of IPv4 address space to be available for under a policy along these lines but I think if it is more than just one or two it probably needs to be designed in a way that takes account of historic demand and how people will react when other avenues are cut off.
Because this is just for initial allocations, I think 2 to 4 years is a reasonable timeframe. IPv4 will run out. This is only meant to make sure that new entrants have a few IPv4 addresses to work with. - Sander