Hard to say the discount rate to apply, but I’d think that almost no discount rate would ever be lower than 1% per decade and if there’s any inherent risk in a proposal that it would need to be at least 5% per decade.

When you talk about “this specific initiative succeeding”, it’s probably more like 1% per year as a minimum. (“Will SpaceX [or its direct descendants] colonize Mars?” is a much less certain proposition than “Will anyone from Earth colonize Mars?” and even the latter is in serious question.)