How long does it take to learn to lead?
There is no definitive answer to that. If anyone tells you otherwise, think twice about their response.
Leading is an art form, and it requires an incredible amount of climbing experience - stress management, decision making, route finding, rope management, gear placement, anchor systems, climbing technique, rescue skills, etc. to be brought together all at once.
You need to have a good head on your shoulders, and it helps to be mechanically inclined. It ranges from person to person; some people can pick it up over a season, some people need a few years, some people may never be able to. The final decision is up to you, and you need to use your judgment. We try not to give instruction to those without adequate experience. How much is required depends on the individual. You should have a good base of knowledge, be able to set up natural and artificial top ropes on your own, have followed a fair amount of lead climbs (preferably multi-pitch), and be comfortable when climbing.