Hi, MC_1, Welcome to the forum! Though you've been here a while, I see this is your first post.
I think the ballpark would be Irun to Oviedo 20 days (
https://www.gronze.com/camino-norte)
Oviedo to Lugo 9 days. (
https://www.gronze.com/camino-primitivo)
But as you can see if you look at the links, those more or less "standard stages" include a lot of short days, so you can probably cut that down if you want. I am hoping to walk the Norte and have tentatively pencilled in 18 days to Oviedo, including a detour day to walk the beach at Santander, and a detour to visit the church at Valdedios.
I always like to give myself some wiggle room, but that's mainly because I would rather do anything than have to catch a bus or train for a stage or so because I didn't give myself enough time. Is this your first camino? Buen camino, Laurie