Boiled linseed oil on leather takes approximately 1.67954 million years to dry!
It soaks into the leather, where air can't reach it (in your lifetime, anyway) and therefore it can't 'dry'/harden by oxidation.
Raw (unboiled) linseed will be the same, only slower.
And while it's drying ( and a new Ice Age is forming in Arizona), it will be sticky as all get out.
Bottom line, don't use linseed (or any 'drying oil') on leather...unless it's gonna be an heirloom for your great-great grandchildren.
Been there, done that...a piece of 1/16" thick leather I used for a tang-wrap took 9 months to quit being sticky.
Ther are, however, 'driers' for linseed oil...generally cobalt naphthenate ("Japan Dryer" at Home Depot) which can speed up the oxidation process by catalysis. They might work, but can turn the oil dark, and because they're oxidation catalysts, may cause the leather to deteriorate. Haven't tried them on leather....yet.
Shoe polish, thinned with lighter fluid, is a better 'poor-man's' leather dye...