I never really did a lease in between taking lessons and buying my horse. At the first barn where I was taking lessons at age 11 I was to start leasing a horse, but I broke my leg (non-horse related) and wasn't able to ride so we cancelled the lease. The barn owner (NOT the lady who owned the horse- she was a girl away at college) was very misleading to the 11 year old child and her non horsey mother when she said I could ride the horse "whenever I wanted". I went out to ride the horse once, and the barn owner was upset that I didn't make arrangements to ride! That is the extent of my leasing experience, and then the leg broke. Once I got back into riding after my leg healed we had issues with the barn so I switched to another about , and began lessons there. After years of regular lessons my parents bough me a horse at age 15, and I pretty much knew the ins and outs of horse care at that point.
Note that I am all for leasing a horse before taking the plunge and purchasing a horse. I do think it is beneficial, but not a necessary step before buying!