I think lots of us have seen the 10,000 hours to master anything complex quote. For a casual trainer say 2 classes/3 hours a week, that's about 64 years
But mastery and being proficient at something are 2 different beasts.
A lot of techniques in Aikido are in other arts, and I think you could learn to apply some techniques very effectively (in terms of applying in a deceive manner) in a short time if that's what you concentrated on. I think what takes time is honing the techniques in terms of blending with different opponents, and efficiency in the use of strength and movement (the -do vs the -jitsu if you will).