For a one stride distance it will be 24 foot
For a two stride distance it will be 36 foot
Basically is 12 foot for take off and landing and 12foot for every canter stride - this is for senior BSJA, not sure about junior BSJA
ETA: Sometimes the course builders will build a distance a bit long or short, so you have to know whether to hold your horse or push your horse forward down the distance, all depends on the individual horse
There is a very useful booklet available from the bsja. (notes on course designing)
This has tables for horse and pony distances and they do vary depending obstacles in your combinations for example upright to upright one stride minium is 24' max is 26' 2 stride 34'6" to 36' where as paralel to paralel 1 stride min is 24'6" max 25'6" and 2 stride is 34'6" to 35'6". the booklet will also give you measurements for related distances both indoors and out.