Mark, normal heathy young person can donate every 6 weeks with common blood groups.
Scarce groups get called as and when required.
When I hit the big 70 it moved to 3 months. The 'elderly' take a lot longer to replenish (despite all the alc to boost production

)
They can also be full of sh1t - once had a lump cut out of face below one eye, stitches were already out, but no come back in 3 months you lost blood in the operation! FFS woman, it was not even under local in the rooms and hardly any blood.
So good buy for 3 months. No use trying another branch as your record is on their computer system!