Aspirina, I'm going to operate under the assumption that you have been in pyro for awhile and have experience with perchlorate-based 7/3. If this is not the case, then please do not even consider making potassium chlorate-based flash. In addition to enhanced friction sensitivity vs perc, chlorate mixtures are also much more sensitive to shock (e.g., dropping) than perc mixtures.
That said, potassium chlorate was used for quite a long time before the safer perc became available. I have made potassium chlorate-based flash before, primarily to get rid of my chlorate stock. You need to appreciate the dangers involved and protect yourself accordingly. Or just don't do it.
Regarding stability, I haven't stored chlorate-based flash, but others, on this site included, have reported no loss of functionality after months or perhaps a year in storage.
Now please explain the addition of boric acid, because that is confusing me. Boric acid is typically added to nitrate/aluminum mixtures to reduce the possibility of unwanted/unanticipated heat generation due to the reaction between aluminum (generally more common with flake vs spherical) and nitrate, or alkaline impurities in KNO3, that generate an exothermic reaction. I cannot see any benefit of adding boric acid to a chlorate- or perchlorate-based 7/3 flash, for either functionality or storage purposes. That said, I of course do not claim to know everything about every formulation--like everybody, I am continually learning new things. This might be the case here, too. So please clarify the reason for the boric acid inclusion.
Take-home message: Be very careful, please. And fully understand the role of every single chemical in any given pyro formulation--if you cannot, then it is premature for you to be working with that formulation.