Air Japan has in the past put almost every First Officer through the upgrade process within the first 5 year contract. I believe a handful had to wait till just past 5 years (due to a backlog at the time). Some have not got through the assessment phase, the majority being given another go a year later and getting through. Generally the success rate for f/os upgrading is pretty high, I would guess well over 90% and this all happening within the first 5 year contract.
As all financial advisors say: "past experience is no promise of future results" I believe AJX does not guarantee this to new f/o's, so there is a chance things may slow down. I guess this is a chance one has to take. The present Capt to FO ratio is around 70:30 right now. It would appear the company prefers to be heavy with Capts, so a typical 50:50 ratio seen in other airlines is not the norm here.
As far a experience for the upgrade WITHIN the company goes, the published requirements for DEC do NOT apply. Every F/O holds a Japanese ATPL, which is the minimum required for a command.
Many of us have achieved our first commands here, so that is not a factor.
I would be looking at the lifestyle this job offers, time away from home and the family, the effects of a commute and time zone changes on you, most relevant to those living in the Americas and Europe.