Time between repeated coats of linseed oil on terracotta

Not a terrible problem with terracotta, but I'd generally suggest avoiding linseed for an application like this as it yellows very powerfully with age. Use tung oil instead, bought ready mixed (don't use pure tung) as "finishing oil" from a reputable supplier like Liberon, Bolloms et al.

I'd also probably go for another resin altogether (the vast range of Lithofin-like products) rather than a single vegetable oil.

As to drying time, then of course It Depends. It depends on heat, humidity, and the drier chemistry used. Some driers work best when moist, some when dry. The usual non-lead driers prefer dry, so that might be a real problem for damp masonry in our recent weather. A "couple of days" can easily shift to "a couple of weeks", at the same temperature and air humidity, just because there was some residual moisture in the terracotta. For that reason the "try a drop on glass and leave it next to it in the workshop" test doesn't work on porouis materials.

In practice, I'd wait until it was surface-dry and then wait until twice that.

If the surface never dries but remains tacky, then you applied it over- thick. Scrub it with white spirit to take the surface goop off, let it dry and then re-apply more thinly.