We used about 35 gallons of linseed oil for our 500 foot sqare earthen floor. This was over the
course of 4 or 5 coasts. It is harder to say specifically how much 'per coat', as the cob absorbs the linseed oil much more rapidly at first. I would reccomend thinning the linseed oil with citrus thinner for the first coats, to get deeper penetration. Maybe like 5:1 ratio. For the final coats, we added beeswax to the linseed oil, which I would reccomend.
We sourced our linseed oil raw, from a company called Montana Vegetable Oil. This was 8 years ago, and they may not be in business anymore, I havent been able to find them lately. The oil came in 5 gallon buckets and was vastly more economical than purchasing in smaller quantities from natural building type suppliers.
There are true experts on this stuff though, unlike me, and I would definitely consult with one before you begin. Lots of ways for things to not quite turn out right.