Most contracts for non spec/education related trades are 3 years. Some trades now have a VIE or BE (Variable engagement or basic engagement) of 4-5 years.
If you want the exact time frame for engagement in a certain trade, I would contact your RC and ask them the questions you have.
But that being said, I wouldn't worry about how long the engagements are, you are joining I presume to have a life long retirement career, and unless you are a causing trouble in your trade, after your initial engagement you will either be offered another BE or an IE25 which means you sign a contract for 25 years total service. They use to offer a IE20, but that was phased out years ago now I believe. Been a while since I was an RMS Clk and policies change regularly.