I'd hope it takes far less than that anyways...
The most recent examples of renderer integration to Gaffer are Arnold and Appleseed. Image Engine integrated Arnold over the course of a summer with a team of 3-4 devs (and several users testing actively in production). That also included fully pipelining all our internal workflows, so the pure Gaffer side took a bit less actually. But then again, we were all familiar with the codebase already, and had an old GafferArnold prototype to start from, so that estimate would probably be conservative for another team.
As far as I know, Esteban at AppleseedHQ took on the Appleseed integration as a one man operation. He was asking questions about approach throughout the process, which is totally welcome, but did all (or most?) of the work on his own. Est, do you have a rough guess how much time you've put in there? Its worth pointing out that as he's part of the Appleseed dev team too, he was able to bend the renderer to fit Gaffer workflows where necessary. I don't know how much that came up in practice, but its a nice card to have in your back pocket.
If you're serious about taking on the Redshift project, I think it'd be a great addition to Gaffer. Another thought, if you're concerned about taking it on purely on your own, would be to reach out to Redshift themselves and see if they can devote any additional resources to the effort. They seem pretty keen to provide plugins provided they have customers requesting them.
Cheers,
Andrew