Anything less than 12 weeks was always accepted as not a real spell, however it can also depend on why the horse was spelled, the usual "spell" where a horse goes to a paddock for a genuine Holiday is 4 weeks in the paddock, it then takes around 8 weeks to get them ready to race again, young horses will generally need a months break as they will have aches and pains/Shin soreness, growing spurts etc, but they can also be up and ready to run in a slightly shorter period, as they race over short distances and they run on Adrenalin so they can work a lot of the fat off themselves, as we all know it is easier to lose weight and get fit when you are 20 than it is when you are 50
sometimes older horses can need a month in the paddock due to stresses that occur, and then take 8, 10 or maybe 12 weeks work to get ready to race, meaning they are out for 4 months or a bit longer, on the other hand a couple of weeks in a paddock would suffice if the horses had no physical problems going out and they could get the same beneficial spell.....they would/could also comeback with some residual fitness so the work they need to be ready to race would be less and the "spell" could total 8 weeks, i.e. 2 weeks in a paddock and 6 weeks work, it would still be a spell.
I am just generalizing, there is NO definitive answer....unless you accept that a spell is the same length as a piece of string