The typical amendment passes within a couple of years.

The 26th - voting age to 18 - was ratified in about 100 days.

The 27th - about Congressional pay raises - took almost 203 years.

Including the 27th Amendment makes the average ratification time almost 9 1/2 years. Take that out and it's about 21 months. (I SWAGed those calcs using 30-day months and did not account for leap years. )