It takes 10,000 hours to mastery, according to Gladwell

http://en.wikipedia.org/wiki/Outliers_(book)

The math:

At 2 times per week = 4 hours per week x 50 weeks = 200 hours per year.

Being generous, at 4 X week = 10 hours/week = 500 hours per year, it would take 20 years to 'master' aikido. I figure that would be about 5th dan level....seems about right to me.

You need to decide what you want for yourself.