The minimum for promotion from 6th to 5th is 30 sessions in my group, and 70 for 1st kyu to shodan. Most students only take 1 or 2 classes a week so they end up taking 4 1/2 to 6 years anyway. However, I'm taking more classes and I'm also a fast learner. I'm not purposely rushing this and i don't feel like I'm going too fast, but I'm a bit worried that I'm learning less.
Another question: when are jo and bokken practice usually introduced in aikido? I'm already being taught the basics by my sensei even though these aren't tested until 4th or 3rd kyu.
You will not be learning less, you might get a little frustrated when you get on and see people put in less effort getting equal 'reward'; but even then you will learn a little humility ...
Bokken / Jo: These can be introduced at any point, I know of an association that test from the get go whereas others don't consider this until a lot (read dan grade) later. You should, IMHO, learn a bit of weapons early on as they teach distance, timing, and a multitude of other things ...