Generally, there is a “category freeze” (aka seat lock) of 2 years before you can bid a new aircraft. There are a few exceptions, like a new hire can bid to change fleets after 1 year if they need to change aircraft to hold a different base. There are also shorter seat locks for shorter courses.

Also, your first Captain upgrade will break a seat lock, but the new “freeze” gets added to your remaining freeze.

Keep in mind, that seat lock is for bidding. Conversion can be anywhere from almost immediately to 150/210/335 days later depending on some variables. The default co version window is 150 days. Though very uncommon, it’s conceivable an award on a 335 day bid it could leave you nearly 3 years in the “old” aircraft, or a new hire nearly 2 years.