Senior people will still learn the domain unless you're hiring them to do something they already know and/or is trivial. I think even for the most senior it's

Senior people will still learn the domain unless you're hiring them to do something they already know and/or is trivial. I think even for the most senior it's like 6-12 months to really come up to speed in a new role and then over the next few years there's still growth that is related to the growth of the project. The senior person is still going to be a lot more effective 3-5 years in assuming you can keep them engaged.