- First and foremost, why are these races ageless? That much implies a mechanism for cell replication notably different from that in H.Sap.
- Consider the microbiology you're using, and adjust the consanguinity ramifications appropriately. If these people cease to age, is it because their DNA doesn't deteriorate through repeated replication?
- Overall, I would think that the lack of ageing implies that these races have far fewer errors in their DNA, and a far lower incidence of error over time. Perhaps this cleanliness derives from a less forgiving reproduction process, such that mutations are almost always fatal.
- As Separatrix pointed out, one would expect these people to have genetic testing. If not, this also suggests that they don't have a reason to worry about their genetic health. Again, reinforcing bad genes isn't a problem for them, eh?
Keep firmly in mind that our "cousins" rules for consanguinity is merely a gross heuristic for risk of reinforcing unfavourable alleles. In order to make this a valid question in SF, it seems that you'll need to engineer a background that avoids the above objections: these people do have a problem with reinforcement, but they have yet to develop research into their own reproductive material, despite enjoying incredibly long lives.
Is there some other endeavour more important for every last member of the race?
That given, we still need the general parameters for the problem: how many chromosomes (or equivalent) do they have? What proportions of undesirable material do they observe? What level of risk is acceptable to them?
Once you decide those parameters, just do the math ... or is that the crux of your problem?